Performance tuning and some other stuff

Performance tuning:

- Removed the Mutex lock for writing. The pipeline already creates an atomic transaction.
- Added an optional parameter validate_field, that enables/disables field validation when saving a model. Disabling the validation makes the write process almost 25% faster.

Removed the Key object. Using a unicode subclass with dict syntax to append strings is counter intuitive. And in many cases those strings can be preprocessed.

Added a main file inside tests, to help running them from command line.