Allow testing with gccgo
Add benchmarks with both gc and gccgo

benchmark on a Lenovo Thinkpad T520
Linux t520 3.8.0-19-generic #30-Ubuntu SMP Wed May 1 16:35:23 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
compiled with gc
Intel(R) Core(TM) i7-2760QM CPU @ 2.40GHz
Add not to README.md about how to set the compiler.

This is relevant to pull request #17.