This patch cleans up a number of issues with how compare_bench.py handled
the command line arguments.

* Use the 'argparse' python module instead of hand rolled parsing. This gives
  better usage messages.

* Add diagnostics for certain --benchmark flags that cannot or should not
  be used with compare_bench.py (eg --benchmark_out_format=csv).

* Don't override the user specified --benchmark_out flag if it's provided.

In future I would like the user to be able to capture both benchmark output
files, but this change is big enough for now.

This fixes issue #313.