Rename main() and fix import

Calling the function main in the module main is just asking for
trouble. The

    from ..cli import main as cli_main
    cli_main(args, env, logger)    # module not callable

is one such instance of trouble.
Delete the YAML C backend

This would require compilation of the _yaml extension module. Since we
aren't going to do that, the cyaml.py is just dead wood. Also, makes
nosetests --with-doctests fail.
fix doctest in jsonschema
Merge pull request #252 from vbraun/doctest

Doctest fixes