No varargs

This removes all use of varargs from the halide_runtime, with the exception of calls to android and ios logging functions. This solves nacl linking issues and some windows mess (e.g. we couldn't print floats properly on windows).

Ready for comments.
This branch doesn't use error(), though both branches of this if are in a failure case. I think either the last print() or both print()s should be error() instead.