Add SQL Backend support

[adewes]

Add support to store and retrieve documents from relational databases such as Postgres or MySQL.

How to do this:

• The table layout will be such that the serialized JSON document gets stored in a BLOB field, and all indexes on values in that document will get stored in additional, indexable columns that have to be defined beforehand.
• Querying is only possible using indexed fields.
• When updating documents, indexed values will get updated automatically.
• When specifying indexes, a type must be given (e.g. text, string, double, int, ...). When trying to store a document with an invalid index value, an exception will get raised.

This way of indexing and storing documents makes it possible, to perform advanced queries on them and even make use of SQL JOIN operators to enrich documents of a given collection with data from other collections, based on some index value in the document.

[dmytrokyrychuk]

PostgreSQL has native JSON support. http://www.postgresql.org/docs/9.3/static/datatype-json.html