Run build tables post install and when building source or binary distribution

Add a post install task to execute `_build_tables.py` in the install destination. This avoid avoids `lextab.py` and `yacctab.py` files being created where pycparser is used when installing from source and forgetting to run `_build_tables.py` manually. As an added bonus, also run `_build_tables.py` automatically when building a source or binary distribution.

Closes #5