Use SQLAlchemy [WIP]

work in progress for #61

I'm fairly new to python but open for comments.

* [x] SQLAlchemy adapter
* [x] Working with current SQLite
* [ ] DB-independent queries
* [ ] Migrations
* [ ] Support PostgreSQL
* [ ] Support MySQL

As of now there is a new `database` option in `general` specifying a URI like `sqlite:////tmp/isso.db` but the old `dbpath` is also supported and will be prefixed with `sqlite:///` if no `database` option is given.