Add write_to_file(); --help command-line flag.

shapefile_parser/shapefile.py
	-Implement `Shapefile.write_to_file()`, and fix bugs in all
	existing `Shape`-inheriting classes' `to_binary()` functions,
	which were untested up to this point.
	-Add a `--help` flag to `_handle_command_line_args()`, and
	robustify error-handling for incorrect command-line argument
	input.