Fix converting association options to a Relation

Under Rails 3 when using an association matcher in conjunction with a
submatcher such as #order, we have to take the options on the
association (:conditions, :order, etc.) and convert them to an
ActiveRecord::Relation object. This happens in ModelReflector.

Unfortunately, converting an :includes option was broken. This is in
part due to ModelReflector not having proper tests, but also, the method
that does the conversion to Relation is rather complex and could be
broken up so it can be tested better. In fact I noticed that there's a
lot of stuff in ModelReflector that does stuff around a Reflection
object, so it would be better to define a ModelReflection class that
simply decorates a Reflection. That's what I've done and also added
proper tests.