New database scheme, storing key name inside tables for improving searching

* dbutils.py: Classes for encapsulating database concepts as tables,
              fields, constraints, etc, easing its translation to
              SQL syntax

* connection.py: Encloses new dbutils classes

* collection.py: Changes at inserting documents, storing key name too

* cursor.py: Key name in searching, using new dbutils classes

* MySQL.py: Implementation of SQL methods for query translations