Merged SDK 2013 updates up to 3b9509df78
https://github.com/ValveSoftware/source-sdk-2013/commits/master

* 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.
Better organize build objects. First steps to fixing Python library.
Fixed typo in dynamic light for explosions and compile error in ge_weaponmelee