Use `_tzset()` for non-POSIX Windows environments

When compiling with MinGW (which does not provide a full POSIX layer as
opposed to CygWin) it's better to use the CRT's underscore-prefixed
`_tzset()` variant to avoid linker issues as Microsoft considers the
POSIX named `tzset()` function deprecated

Further reading

 - http://msdn.microsoft.com/en-us/library/ms235384.aspx
 - http://stackoverflow.com/questions/23477746/what-are-the-posix-like-functions-in-msvcs-c-runtime

This hopefully addresses #2
Merge pull request #4 from hvr/pr-tzset

Use `_tzset()` for non-POSIX Windows environments