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
* 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 #14 from kynan/run-build-tables-post-install

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