Update to gopkg.in/mgo.v2 repository

labix.org/v2/mgo is no longer maintained.
Allow JUJU_MONGOD environment variable through
Allow specifying path of mongod in $JUJU_MONGOD
Provide MgoInstance flag for enabling journaling
Merge pull request #24 from axw/testing-mongo2.6

Update to gopkg.in/mgo.v2 repository