Backends

After see how is processing processes for ejabberd and the possibility to use another system as mnesia instead of gen_leader we decide to wrap the main functions for forseti and implement a backends system.

The initial backends available:

- **gen_leader**: as is in previous versions. Let you use the gen_leader system to build a distribution process system.
- **mnesia**: copying the ejabberd way, use mnesia (transactions to ensure consistency) and the possibility to use *dirty* for several actions (but not recommended).

Try them and tell us what do you think :-)