Ormar

* Add fields() method to limit the selected columns from database - only nullable columns can be excluded.
* Added UniqueColumns and constraints list in model Meta to build unique constraints on list of columns.
* Added UUID field type based on Char(32) column type.

* Added bulk_create and bulk_update for operations on multiple objects.

Add queryset level methods
* delete
* update
* get_or_create
* update_or_create

* Add additional filters - startswith and endswith

* Add choices parameter to all fields - limiting the accepted values to ones provided

* Added exclude to filter where not conditions.
* Added tests for mysql and postgres with fixes for postgres.
* Rafactors and cleanup.