Add SQL Backend

[adewes]

Currently, Blitz supports a file-based backend and a MongoDB backend. We want to add a SQL backend using SQLAlchemy so that people can use Blitz in conjunction with Postgres, MySQL, SQLite and many other SQL databases!

• Create a new backend folder: sql
• Write a SQL backend that implements all necessary functionality (filter, get, delete, ...)
• Add the backend to the test suite and, if necessary, add additional tests