General:
*         Upgraded Steamworks SDK to v1.29
*         Fixed mod compatibility problem with Multiplayer Base that was introduced in September.
*         In Hammer, while using the Vertex Tool, pressing CTRL+B will snap selected vertices to the grid.

Virtual Reality:
*         Mods that support virtual reality now need to have a line in gameinfo.txt that says “supportsvr 1”. This indicates to gameui and engine that certain UI should be enabled.
*         VR-enabled mods will now start up in VR mode when launched from Steam’s VR mode.

Windows:
*         Upgraded to Visual Studio 2013. If you need to build projects for VS 2010, add /2010 to your VPC command line.

OSX:
*         Upgraded to XCode 5.
Merge branch 'master' of https://github.com/ValveSoftware/source-sdk-2013
Merge remote-tracking branch 'upstream/master'
Added VS2010 version of libprotobuf for folks who are still using VS2010
Update isurface wrapper for changes in source sdk 2013 update.
Merge remote-tracking branch 'upstream/master'
Update generated code for vgui module.