Use process uids and usernames on Unix and Windows respectively.

Windows doesn't have EUIDs, so instead we'll check that the usernames
match. This doesn't seem the least bit secure to me, but it's Windows so
they're probably owned anyway. If anyone knows one of the "proper" ways
to determine if another process has the same owner on Windows, I'd love
to know about it.

 * FIXES Issue #58 but I don't have a Windows machine to test so maybe
   it's still broken.
   https://github.com/isislovecruft/python-gnupg/issues/58
Merge branch 'fix/58-win32-uid' into develop