vim: Preliminary Python 3 support

# NOTE: ~~WIP~~ Almost done (as 20130930)
Summary: vim 7.4 will not build both Python 2 & 3 bindings unless build them with dynamic.

* Force using specific dynamic Python library if necessary.
* Unset unnecessary PYTHONPATH.
* Support external Framework Python (should be with same architecture).

If you have problem, such as window disappear, try `[m]vim --noplugin` first.
Tested with vim 7.4.027
Reference: <http://vimdoc.sourceforge.net/htmldoc/if_pyth.html#python3>

## TODO
* [x] Check 32-bit MacPython.
* [x] Match coding style.