Added ATS to language list and several samples (try #2).

This subsumes @wdblair's pull request https://github.com/github/linguist/pull/828 primarily by adding several examples, and moving the .cats extension to be a C file, as these are actually C files intended to interface between C and ATS.

ATS is a statically typed programming language that unifies implementation with formal specification. It is equipped with a highly expressive type system rooted in the framework Applied Type System, which gives the language its name. In particular, both dependent types and linear types are available in ATS.

Language Website:
http://www.ats-lang.org

Latest Compiler (also includes many software projects in the contrib subdirectory):
http://github.com/githwxi/ATS-Postiats

Some other github projects using ATS (I'm sure I've missed some, and my laptop battery is about to die, so this is it for now.):
https://github.com/githwxi/ATS-CMake
https://github.com/bbarker/FALCON
https://github.com/doublec/ats-task
https://github.com/doublec/ats-libzmq
https://github.com/doublec/ats-libevent
https://github.com/doublec/ats-jansson
https://github.com/doublec/ats-bitcoin