Add additional filters - startswith and endswith

Like described in encode/orm#49 - additional filter operators using like and ilike under the hood

Add choices param

similiar to one described in encode/orm#57

Added exclude, more fixes and tests

Added exclude to filter where not conditions.

Added tests for mysql and postgres with fixes for postgres.

Rafactors and cleanup.

Added Many2Many relations

Merge pull request #4 from collerek/many_to_many

Many to many