Move to mongodb offical driver (apache#22)

* move for offical driver

* Remove useless dependencies

* return panic error

* customize the databaseName

fix database name

* Use alias for net/url

Merge pull request apache#20 from adamwasila/casbin_v2

Update to use v2 major version of Casbin

Merge pull request apache#18 from adamwasila/add_modules

Add modules files