Updated help (and more robust argument parsing)

[jashank]

The help message is somewhat out-of-date --

Usage: dcc [--valgrind|--memory|--no_explanation] <c-files>

-- as dcc supports more flags. But, to mitigate this problem and to improve the somewhat impromptu argument parsing, switching to something like argparse should improve matters.

[andrew-taylor]

dcc has to pass through clang arguments which makes using argparse problematic

help message has been update