Add basic support for SQLite

[jmisasa]

Hi @jhartwell

I am currently learning Elixir and Plsm has come in handy to generate Ecto models from an existing basic SQLite database of a pet project that I want to migrate to Phoenix as practice.

Since it might also be useful to someone else, here are the proposed changes.

One of the caveats is the lack of support for foreign keys. I'd be glad to read your thoughts and address your proposed modifications.

Thanks for your work on Plsm!