Add sym_get to CodeGen interface
Store buffer_ts in symbol table for GPU backends
Add CodeGen helpers to write buffer_t fields
Document low-level CodeGen_Posix malloc interface
Merge branch 'master' of github.com:halide/Halide
Switch to use a per-compile LLVMContext

This resolves issues with all the types and global symbols 
getting mangled names on successive compiles, to maintain 
unique IDs throughout the (previously shared, global) context.
Add JITed device buffer management hooks to Buffer
Initial C++ PTX backend
PTX backend build fixes for Linux
Further build fixes

(no idea how this works fine on OS X)
Compilation fix for trunk llvm
Add SIMT loop branches to PTX_Dev
Trivial initial GPU test (bounds)
Merge branch 'master' of github.com:halide/Halide
Compilation fix for linux
Fix runtime.ptx_host build on Linux
Merge branch 'master' of github.com:halide/Halide

Conflicts:
	cpp/src/runtime/runtime.ptx_host.cpp
Refactor JITCompiledModule support for platform specialization

Add lazy CUDA symbol initialization hooks
Merge branch 'master' of github.com:halide/Halide

Conflicts:
	cpp/src/CodeGen.cpp
	cpp/src/CodeGen.h
	cpp/src/CodeGen_Internal.cpp
	cpp/src/CodeGen_Internal.h
	cpp/src/CodeGen_PTX_Host.cpp
	cpp/src/CodeGen_PTX_Host.h
Fix Linux build: switch to cerr