ENH: use proper uri instead of hacked-up string manipulation for repos.

This allows us to fix quite a few windows test failures/errors.
BUG: fix uri_to_path and add tests.
BUG: fix tests to keep platform-specificities within platform guards.
Merge pull request #104 from enthought/fix_win32_tests

ENH: use proper uri instead of hacked-up string manipulation for repos.