New Mutex class using transactions.
Add a new "Collection" object to allow dynamic import of classes in _set functions
Add forgotten import
* #14 ``db`` object is fetch from the ModelOptions if provided
Update doc for #14
Implement the ModelSet access with Meta: db. references #14.
fixes #13
Merge pull request #8 from jeloou/master

Using transactions in Mutex.
added url parameter
Merge branch '0.2' of https://github.com/asa/redisco into 0.2
url redisco setup