Find an appropriate Python executable for libuv build

Fix for #66.

There are more edge cases than I initially thought. Here are some points I considered:

1. I'm not doing the check exclusively on Python 3 builds because the user might have started the build with an absolute path to Python (e.g. `C:\Pyton27\python setup.py ...`)
1. Before checking default installations there is the test if the Python on PATH is already fine.