Fix Vs2012 naming

It was inconsistent with previous VS version naming.
Remove VS2012 Debug Configuration
Add VS2013 support
Fix incorrect registry path and method name
Enable VS2013 client build in Debug|x86.
Alias GitTfs.vs2012 and GitTfs.Vs11 (when referenced from GIT_TFS_CLIENT).
Extract a base class for VS2012 and VS2013.
The VS2012 and VS2013 Workspace.Checkin methods are the same, so use the same code.
Merge pull request #464 from DanRigby/VS2013

Add support for VS2013 & TFS2013