MongoDB test layers

Hi lovelies,

during christmas/newyear's time i got around to establish test layers for MongoDB. Hope you like it::

    bin/buildout install mongodb mongodb-test
    bin/test-mongodb

There are three flavors of test layers:
  - MongoLayer: Just a single MongoDB instance
  - MongoMasterSlaveLayer: Two or more MongoDB instances configured in a master-slave setup
  - MongoReplicaSetLayer: Two or more MongoDB instances configured in a replica set setup

For running tests of individual scenarios::

    bin/test-mongodb --test=mongodb_single
    bin/test-mongodb --test=mongodb_masterslave
    bin/test-mongodb --test=mongodb_replicaset

Cheers,
Andreas.