Add "examples" section to extras_require in setup.py

[aaronayres35]

This would make it so that one could do pip install envisage[examples] and be assured that they have all the dependencies needed to run all of the examples.

This may seem like a simple issue, but because chaco is a dependency, which depends on enable things are a little muddy. In order to install enable (more specifically kiva), you need to already have numpy installed in the environment, and further you need a specific version of swig.
Thus, simply adding "examples": ["numpy", "scipy", "chaco", "mayavi"] to extras_require will not work. We will need to do a bit more.

[aaronayres35]

This should be a bit easier soon after the next enable and chaco releases.
see PRs:
enthought/enable#808 enthought/enable#811 enthought/chaco#730

specific swig version is still a little clunky but with those changes, I think we should be able to address the issue