Issue144

[darthunix]

A pull request, closing issue144 and closing TODOs in constraint.py.

• on index columns change drop/add index
• on primary key columns change drop/add primary key
• on foreign keys columns/match/actions change drop/add foreign key
• on unique constraint change drop/add constraint

[jmafc]

@darthunix In future PRs, you may want to aggregate commits, e.g., use git commit --amend when you notice the "ooops" or use git rebase --interactive to squash several commits. Having fewer individual commits also makes it easier to merge changes to other branches.

[darthunix]

Thanks for advice. I promise to read git cookiebook on weekends)