OS X ABI tag for Python 3.5

[Min RK]

Python 3.5 on OS X has changed SOABI from 'cpython-34m' for 3.4 to 'cpython-35m-darwin'. Hyphens in SOABI are not considered in the code, resulting in a couple small but significant bugs: wheel is producing the tag `cpdarwin`, which is wrong, and pip is expecting the tag `cp35m-darwin`, which is invalid due to the hyphen. It should be easy enough to fix in both packages, but it's not quite clear what the right answer is. Should it be:

- 'cp35m' like it was for CPython 3.4, or

- 'cp35m_darwin' - include the whole SOABI, but normalize `-` to make it a valid tag.

I have a patch for pip and wheel to do the first option, which makes the most sense to me, but the second is easy enough if that's preferred.

Yet another option would be to revert the `-darwin` suffix in SOABI, but that's perhaps going too far.

-MinRK