Complex filters in style buckets

[mourner]

Implements #407, MongoDB-style except the differences discussed. Covered with tests.

{
  'foo': {'>': 5, '<=': 16},
  '$or': [
    '$and': [
      '$not': {'bar': 'baz'},
      'foo': {'!=': 10}
    ],
    'cool': {'$exists': true}
  ]
}

Operators implemented:

• $lt (<), $gt (>), $lte (<=), $gte (>=), $ne (!=), $exists
• $or (||), $and (&&), $nor ('!': [...])
• $not ('!': {...})

[mourner]

The build fails because of @tmcw's recent tests — safe to merge.

[ansis]

Nice implementation!