Run post install task executing _build_tables

* 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
Merge pull request #20 from kynan/build-tables-post-install-task

Run post install task executing _build_tables