Add docs and examples to the source distribution

[jwiggins]

Fixes #213

[jonathanrocher]

I tried to build the html docs yesterday from the sdist I generated, and failed with the following error because of the way conf.py looks for enable's __init__:

$ make html
sphinx-build -b html -d build/doctrees   source build/html
Running Sphinx v1.2.3

Exception occurred:
  File "../../enable/__init__.py", line 6, in <module>
ImportError: cannot import name full_version
The full traceback has been saved in /var/folders/ff/z56qhb_936g8xx3rzm1_1r800000gp/T/sphinx-err-gn5R7X.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://bitbucket.org/birkenfeld/sphinx/issues/>. Thanks!
make: *** [html] Error 1

We may want to fix that in conf.py...

[jwiggins]

Nice catch @jonathanrocher. I've changed conf.py over to enable/_version.py and it appears to work.