Allow queryable defaults to be set via @queryable_defaults, closes #127
Merge pull request #128 from jeregrine/migration-readme

Updating Readme Docs for Migrations
Removing nil keys and values from insert cluases
Forgot to run integration tests. Ran and made updates to accommodate.
Fixing some syntax errors, returning multiple values if nil
Fixing unit tests, altering Postgres.create to return an updated entity