Create v0 and experimental namespace for python

[yongtang]

This PR is part of the effort to clean up the namespace
for python. Quite a few files have been changed but the only real
changes are:

1. Created a namespace of tfio.experimental for future usage
2. Created a namespace of tfio.v0 to indicate the current release namespace
3. Alias all tfio.v0 namespace to tfio, so that no end user impact.
   For example, tfio.IODataset.from_audio is an alias of tfio.v0.IODataset.from_audio
   and could be used concurrently.
4. Move LibSVM to tfio.experimental.IODataset.from_libsvm and tfio.experimental.text.decode_libsvm, to setup an initial example.

Overall, this is a poor man's version of API namespace management (compared to core tensorflow's API namespace management).

Below are the output of APIs. As we could see API namespace has been much cleaned up:

# TFIO_DATAPATH=bazel-bin python
Python 2.7.12 (default, Oct  8 2019, 14:14:10) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import tensorflow_io as tfio
>>> dir(tfio)
['IODataset', 'IOStreamDataset', 'IOTensor', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__', '__version__', 'core', 'experimental', 'v0', 'version']
>>> dir(tfio.version)
['VERSION', '__builtins__', '__doc__', '__file__', '__name__', '__package__']
>>> tfio.__version__
'0.10.0'
>>> dir(tfio.v0)
['IODataset', 'IOStreamDataset', 'IOTensor', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__']
>>> dir(tfio.experimental)
['IODataset', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__', 'text']
>>> dir(tfio.experimental.text)
['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'decode_libsvm']
>>> 

This PR is part of the discussion in #553.

Signed-off-by: Yong Tang yong.tang.github@outlook.com

[yongtang]

@terrytangyuan @BryanCutler This PR is ready for review. Would you mind taking a look and see if the api namespace layout makes sense?

[terrytangyuan]

Looks great!

[BryanCutler]

SGTM!