Prevent two classes with the same name.

This is a temporary fix. This code should probably be factored out.
Use the same ZeroGPULoopMins class from OpenGL and generic GPU code.
Export zero_gpu_loop_mins() function instead of IRMutator
Merge pull request #346 from halide/zero_gpu_loop_mins

Use the same code to zero GPU loop mins from OpenGL and generic GPU code.
Moved CUDA declarations to mini_cuda.h.
Add a test that uses print and print_when.
DebugToFile was crashing on extern stages

Fixes #344
Merge branch 'master' of github.com:halide/Halide
Merge branch 'master' of https://github.com/halide/Halide
Merge branch 'master' into opengl-android