Adding support to MailKit for Windows 8 and Windows 8.1

Currently only IMAP is working (for both new platforms).

There are comments for things I wasn't sure about, but I've been testing IMAP and it's all working great so far!

Even though POP3 and SMTP won't work for Windows 8 and Windows 8.1, they'll still work for all the other platforms (none of their code was touched).