Support mongo datastores

.travis.yml

README.md

cmd/cashierd/main.go

cmd/dbinit/dbinit.go

server/store/config_test.go

server/store/mongo.go

server/store/mysql.go

server/store/store_test.go

vendor/gopkg.in/mgo.v2/LICENSE

vendor/gopkg.in/mgo.v2/Makefile

vendor/gopkg.in/mgo.v2/README.md

vendor/gopkg.in/mgo.v2/auth.go

vendor/gopkg.in/mgo.v2/bson/LICENSE

vendor/gopkg.in/mgo.v2/bson/bson.go

vendor/gopkg.in/mgo.v2/bson/decode.go

vendor/gopkg.in/mgo.v2/bson/encode.go

vendor/gopkg.in/mgo.v2/bulk.go

vendor/gopkg.in/mgo.v2/cluster.go

vendor/gopkg.in/mgo.v2/doc.go

vendor/gopkg.in/mgo.v2/gridfs.go