Merge pull request #24 from erikporter/master

Adding support to MailKit for Windows 8 and Windows 8.1
Fixed indenting
Fixed indenting
Fixed indenting
Simplified Windows 8 logic by using a DuplexStream
Added MailKit.Security.AuthenticationException
First attempt at implementing SMTP and POP3 support for Windows RT 8
Moved DuplexStream into the toplevel so it can be shared among SMTP, POP3, and IMAP
Fixed Pop3 authentication unit tests (due to new exception type)