Using argparse?

Now I realize that argparse was probably left out on purpose for 2.6 support. However, `future` kind of, sort of, requires argparse (only really needed for `http.server`). But I suppose now's as good a time as any to think about switching.

The main benefit of argparse is it will handle positional arguments, so something like the `obspy-flinn-engdahl` script can specify that it uses longitude/latitude, then it will appear in the usage and --help, plus argparse will test for them. Also, there's some simplification of default values, etc.

This PR is not 100% complete; I did not convert the three largest scripts. I wanted to gauge interest and see if there were specific concerns about this before tackling the big ones.