Added profiling targets to Makefile. 

I've added a set of targets to the Makefile that can be used to generate profiling data for all the tests included with Halide. The main target profile_tests will run all tests and use the unix time command to gather runtime statistics. For most tests the run times include Halide JIT compilation as well as runtime execution of the generated function. The generator tests are profiled by timing the AOT compilation, and so provide timing for the Halide compilation only. Results of the timings are collected in csv files for later analysis.  
Could you add just a brief comment explaining that this (mostly) measures the timing of the compiler itself, and not performance of our generated code? I realize it sounds silly/unlikely but I want to avoid people confusing this for benchmarking data.