Adds unicode support to Windows environment.

[sipsorcery]

Currently the Windows environment uses the *A ANSI Win32 API calls for file system operations. This precludes the ability of consuming applications to pass paths with unicode characters. A detailed discussion is in #755.

This PR swtiches the *A ANSI calls to the *W wide unicode string calls along with the conversion methods from standard UTF-8 strings to the UTF-16 multi-byte strings requires by the Win32 *W API functions.

[maflcko]

Needs rebase?

[jonsimantov]

Hi, is there any update to this PR? It would be very helpful if it were merged.

[cucumber12121]

ping