Make thrift a submodule

Summary: This diff makes thrift a submodule, instead of copypasta'ing it in.

CMake only builds the stuff needed under thrift/lib/cpp (probably still more than hhvm needs).

Test Plan: built hhvm.

Conflicts:
	folly/CMakeLists.txt
Merge pull request #34 from djwatson/master

Make thrift a submodule