Fix support for MongoDB

[neithere]

Addresses issue #80.

Changes:

• Add package dependency. (Might be optional?)
• Add option --use_mongodb to prefer that backend over Redis. (A mandatory --backend={mongodb|redis} might be better?)
• Use defaults for theme and board size to prevent breaking UI with empty values. (Should be on a higher level but for some reason it works with Redis as is.)
• Return cards as a list of hashes instead of hash of hashes.
• Avoid shadowing incoming argument variables with values of a different type. (Ideally should be either extended to other functions for consistency or reverted if there's some reason for that like a JavaScript idiom.)

[Bisonfan95]

Hello Neit! Did your changes fix the MongoDB support?