Fixed ImapEngine.Inbox for servers that use INBOX as the namespace

Thanks to @joancaron for this fix (issue #37)
Update README.md
Fixed Exception raised (server "BAD" response) when we try to retrieve Message Summaries

It occurs only when "ALL", "FULL" and "FAST" were selected
(MessageSummaryItems enum values)
Merge pull request #38 from joancaron/master

Server "BAD" response when we fetch message summaries
Fixed FormatSummaryItems() to follow rfc3501 more strictly
Added a parser for NTLM's TargetInfo
Added fetching GMail labels (X-GM-LABELS).
Merge pull request #42 from AntonGordienko/master

Added fetching GMail labels (X-GM-LABELS).
Fixed up formatting and some logic for the X-GM-LABELS feature
Use a list of strings instead of IFolders for X-GM-LABELS

The FolderCache will not necessarily have a mapping of all of the
user's folders, so using this will be unreliable. We also can't
send new commands to lookup/get the corresponding folders because
the command loop is not reentrant.

The easiest solution is to simply use a list of strings and have the
app deal with that.
Added support for searching for GMail labels
UTF-7 decode the GMail label names
Bumped verion to 0.12.0.0
Updated MailKit.nuspec
Added more IMAP examples
Make sure that DIGEST-MD5 realms are not null before dereferencing

Fixes issue #45 (many thanks to ckopas for digging into this)
Simplified the API by making CancellationTokens optional params
Bumped version to 0.13.0.0
Updated README.md for X-GM-LABELS status change
Update README.md