Update riak client
Add Ruby 2.1.0 to travis
Create InvalidClass error
Raise an exception when assign an invalid class
Update finders and ResultSet #all method.

Now we can retrieve many objects given an array of ids.
Change generic method name for add an object to a collection.
Change `collection` behaviour.

It is changed to use the most performant way to associate many objects in a NoSQL database. 
Now an array of ids is persisted in order to retrieve all the objects without using an index search.
But it won't change the referenced object, the parent attribute must be set if is pretended to be persisted.

The previous behaviour was the relation-way to do it, using indices.
Do not instantiate Embedded objects on loading.
Silent annoying warnings from I18n (riak-client dependency)
Update README example, the new behaviour of `collection` and a brief description for `index` and `unique`

[ci skip]
Test that `foreign_keys` attributes are listed in attributes hash.
Merge pull request #19 from emancu/change_collection

Change collection behaviour