fix: validate values - inserting (storage)block object into Blocks Object

- val in __setitem__(self, key, val) is now validated with
  MetadataBlockType Rule and StorageBlockTypeRule

- The current functionality shall be in place till
  https://github.com/rackerlabs/deuce-client/issues/26 is solved

- Better Error messages when trying to update storage_id, block_id
  given that the operation is not allowed

- Some Typos and repeated docstrings corrected

- Tests refactored, to make introduce sorted lists of uuids, instead
  of hardcoding them