Check in gitattributes for binaries.

Followup to @TheSeg's comments on 5f08baa.
* Switched the SDK from checked-in projects to VPC, the Valve Project Creator. See the Getting Started document on the wiki for details.
* Pulled in bug fixes from HL2 and HL2MP.
Fixed execute permissions on OSX/Linux scripts and binaries.
Merge https://github.com/ValveSoftware/source-sdk-2013
Updated the libs. Updated vpc_* to include support for $Conditional in .vgc files. Included vstdlib_exclude.vpc for the OSX builds.
Update text & binary attributes.

This should prevent line ending issues.
Normalize line endings

Fixes issue #75.