Perceval-mozilla

**New features:**

* Dependencies file for development\
Include a new requirements.txt file to install the development version
from the repositories.

** New features improvements: **

* Backends updated to be compatible with Perceval 0.11.
* Creates backend added.

** New features and improvements: **

* New set of backends added:
- Kitsune
- Mozilla Club
- ReMo
* Backends updated to be compatible with Perceval 0.5, using the namespace
schema and the new implementation of `BackendCommand`.

** Bugs fixed: **

* Offset was not stored in ReMo cache. This caused errors when trying
to resume with items from the cache. (1)