Added profiling targets to Makefile. Running make profile_tets will run each test under the unix time command and record the results to a csv file. Each group of tests, correctness, performance, generators, etc... will be recorded in its own csv file and has its own target to run directly.
Added a comment to the Makefile describing the profile_% targets.
Renamed the profile_ targets to time_compilation_. The generated files are named compile_times_.
Minor fix to the comments.
Merge pull request #557 from halide/profile-tests

Added profiling targets to Makefile.