Run post install task executing _build_tables

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.

* Add post install task executing _build_tables in install destination
* Use distutils execute for building tables to respect dry_run flag
* Use setuptools if available, fall back to distutils if not
* Name the overriden class install to get the right help text
* Also run _build_tables when creating a source distribution
* No more need to run _build_tables.py manually

Closes #5