add SQL backend

[JosuaKrause]

redipy.sql: A backend that provides the Redis functionality via SQL on traditional database systems, such as SQLite, PostgreSQL, or MySQL. This backend can take advantage of and share some code with the memory backend. Pipelines can be executed as transactions. Scripts can be executed as is with redis function calls inside an transaction. Defining SQL queries to map redis functions can use a similar plugin system to the existing one.