Added empty Collection module

A first (pending) acceptance test has been outlined as well.
Implemented Collection.by_key

Behind the doors there no real implementation yet
Added spec for configuring the Collection
Use a double instead of a fake implemenation
Tackled Collection.save

Only `create_document` no validation etc.
Added timestamps just before creating the document
Setting Model#key after document creation
Setting Model#rev after creating a model
Always return the model after calling Collection.save
Dropped the 'successful creation' context
Calling Model#valid? when saving a model
Adjusted Reek configuration
Refactored Collection#save
Wording of Spec descriptions
Collection Spec structure: order by method names
Implemented Collection#delete
Refactored Collection#delete
Collection#replace sets the updated_at timestamp
Collection#replace replaces the document via the connection
Collection.replace sets the rev of the model