Rudimentary support for Enum

[spkersten]

This PR adds rudimentary support for Python 3.4's Enum type. The only thing it does is setting the type of Color.red to Color given class Color(Enum): red = 1.

While it doesn't check much, it suppresses the error that mypy currently reports for the most basic use of Enum.

[presidento]

Can you add tests for the new functionality?

[spkersten]

@presidento Sure, but I've not time for that now. (I've to figure out how to make a Python 3.4 only test; not sure if that is currently possible.)

Also, the code needs to check that the version of Python is > 3.4.

[JukkaL]

Currently Python version only affects mypy/test/data/pythoneval.test. You can add Python 3.4 only test cases easily by modifying python_34_eval_files in mypy/test/testpythoneval.py.

[o11c]

IMO instead of making it 3.4 only, it should add a dependency on enum34 for older versions.

[JukkaL]

It would be great to get this PR in! Apologies for the delay -- I've been traveling a lot and moving (twice!) in the last month, but things are now getting back to normal.

@spkersten Can you have a look at the test failure?

[JukkaL]

@o11c I created a task for this: #742