Vakt

Added
- [Storage] Irreversible exception for migrations.
- [Storage] MongoStorage migration between 1.1.0 and 1.1.1

Fixed
- [Rules] Failing JSON deserialization of some Rules.
- [Storage] Migration Migration0To1x0x3 was properly renamed to Migration0To1x1x0.

Changed
- [Util] Objects are now serialized via `jsonpickle` library. This affects Rule JSON representation as JSON string.

Added
- [Storage] MongoDB storage implementation.
- [Storage] `Migration` interface for specifying storage migrations actions.
- [Storage] `Storage:_check_limit_and_offset` method for generic limit and offset validation.
- [Checker] `UnknownCheckerType` exception.

Changed
- [Util] `JsonDumper` is now called JsonSerializer.
- [Storage] `Storage:find_for_inquiry` now accepts Checker object as the 3-rd optional argument.

Changed
- Only small documentation bits.

Added
- Initial implementation of Vakt. Only in-memory policies storage available.