Add exp/log/pow to GLSL codegen test
Allow vector arguments for floor, ceil, round
Defer lowering of log, exp, pow until codegen

Change halide_log, halide_exp to work with vectors since they are now lowered
after vectorization.

Move handling of constant arguments to simplifier.
Use anonymous namespace for helper functions
Better comment
Merge pull request #480 from halide/exp_log_pow

Defer lowering of log, exp, pow until codegen