Sifts

This release renames the `SearchEngine` factory to `Collection` and its `prefix` argument to `name`, which is now compulsory.

This release contains performance improvements - some of them drastic - and adds profiling script to assess performance.

New methods `delete_all` and `all_documents` are added.

This release fixes a couple of issues and introduces some API changes.

This is the first working release, supporting SQLite and PostgreSQL as backends.

Initial SQLite implementation.