Relaxing constraint on Rails 4.
Add bridge code to work with Rails 4.
Gracefully handle mass-assignment security.

Mass-Assignment security has been extracted in Rails 4. That means when
it's not loaded, the arity of the persistence methods changes (e.g. they
don't accept options). This is now handled by optionally including a
mass-assignment compatible peristence module.

Also, without mass-assignment security loaded, there
accessible/protected attributes don't exist, so there's no need to
filter them. This handled by optionally including a mass-assignment
compatible transformation module.
Use AS::Concern everywhere.
Lazily map columns.
Update transformation to reference attributes.

It's clearer and more constistent if transformations are converting
fields to attributes instead of columns.
Use instance variables directly instead of attr accessors

Defining an attr accessor in the included block overrides the getter in
the class methods module, so just skip it an use the instance variables
directly.
Autoload protoable into anything that inherits from AR.
Override constructor to accept PB message as attributes.
Add model_scope to handle Rails 4 compatibility.
Include heredity directly.
Add ability to alias protobuf fields.

Add a new DSL method that creates a protoable field alias. Field
aliases create custom converters for mapping protobuf fields to AR
attributes/columns that aren't automatically mapped by protoable.
Require libs that are used for serialization.