Adds a new class `PMapView` to the `pyrsistent._pmap` namespace. This class is
intended to be private and essentially mimics the built-in `dict_values` and
`dict_items` pseudo-types. Under the hood they just keep a reference to the
`PMap` object and use the `iteritems` and `itervalues` methods; this means that
creating a view can be run in constant time insteead of the current `O(n)` time.

The `keys` method of `PMap` does not use the `PMapView` class because `PSet` is
aleady a `PMapView`-type class for `PMap` keys. I couldn't think of a reason not
to just use `PSet`: it's the logical type for a set of keys of a persistent map,
and it can just be passed a `PMap` to create such a set in constant time.

The tests were also updated to reflect the reality that `keys` now returns a
`PSet` instead of a `PVector` and that `values` and `items` return `PMapView`s.

As a benchmark, the following code:

```python
from timeit import timeit

def make_setup(mapsize):
    return ('from pyrsistent import pmap, pvector; '
            'm = pmap({k:k for k in range(%d)})' % (mapsize,))

tests = [('len(m.keys())',   'len(pvector(m.iterkeys()))'),
         ('len(m.values())', 'len(pvector(m.itervalues()))'),
         ('len(m.items())',  'len(pvector(m.iteritems()))')]

for mapsize in [10, 100, 1000]:
    print("Map size:", str(mapsize))
    setupstr = make_setup(mapsize)
    for test_pair in tests:
        print('=' * 60)
        print('%-40s' % test_pair[0], '%6f us' % timeit(test_pair[0], setup=setupstr))
        print('%-40s' % test_pair[1], '%6f us' % timeit(test_pair[1], setup=setupstr))
        print('-' * 60)
    print('')
```

produces the following output on my desktop:

```
Map size: 10
============================================================
len(m.keys())                            1.330579 us
len(pvector(m.iterkeys()))               1.562125 us
------------------------------------------------------------
============================================================
len(m.values())                          0.653008 us
len(pvector(m.itervalues()))             1.584405 us
------------------------------------------------------------
============================================================
len(m.items())                           0.653144 us
len(pvector(m.iteritems()))              1.159099 us
------------------------------------------------------------

Map size: 100
============================================================
len(m.keys())                            1.350190 us
len(pvector(m.iterkeys()))               11.702080 us
------------------------------------------------------------
============================================================
len(m.values())                          0.650723 us
len(pvector(m.itervalues()))             11.739465 us
------------------------------------------------------------
============================================================
len(m.items())                           0.653189 us
len(pvector(m.iteritems()))              8.951215 us
------------------------------------------------------------

Map size: 1000
============================================================
len(m.keys())                            1.379429 us
len(pvector(m.iterkeys()))               114.202673 us
------------------------------------------------------------
============================================================
len(m.values())                          0.679722 us
len(pvector(m.itervalues()))             113.874931 us
------------------------------------------------------------
============================================================
len(m.items())                           0.680877 us
len(pvector(m.iteritems()))              87.645472 us
------------------------------------------------------------
```