parser.py: Add keyword arguments to parse_args()

This makes the definition of the overridden ArgumentParser.parse_args()
method match that of the argparse definition.
ArgumentParser raises exception on err, not exits

This modifies the behaviour of the srtime.ArgumentParser class so that
if an error occurs while parsing args, an ArgumentParserException is
thrown, rather than the default argparse behaviour of printing the error
message and exiting the program.
Add basic args and command test coverage for