Begin field refactor by removing hard-coded DDL bits / templating logic.
Bring back Entity
Constraints.
Small tweaks
Merge branch 'master' into field-refactor
Removing a bunch of crap
Refactoring DDL generation to use node constructs.
Get the rest of the DDL methods.
Merge branch 'master' into field-refactor
Closer.
Merge branch 'master' into field-refactor

Conflicts:
	peewee.py
	playhouse/postgres_ext.py
Fix custom create_index in postgresql_ext.
Cleaning up Sqlite code.
Fix for referencing cols on different models.
Fixing migration extension.
Fixing some dangling bugs.
Fix create_foreign_key logic.
Updating notes.
Updating to-dos.
Test for error with aliased column in raw query.