Improve codec generation from third-party specs

[zinid]

This is a follow-up for #9
The generator should generate separate files (both *.erl and *.hrl) in the third-party project directory. So there should be no hassle to copy anything and compare diffs.
After this is implemented the documentation should be written on how to use the generator. Existing one on the issue mentioned above is not a very good source of information.