Refactor definition_proxy_spec

This introduces a couple of things: firstly, there's now a MockFactory
that allows for introspection instead of spying all over the place. This
should make tests a lot less brittle. Secondly, a couple of handy
matchers have been made available to perform assertions against the
mock.

spec/support/matchers/trait.rb

@@ -0,0 +1,9 @@
+RSpec::Matchers.define :have_trait do |trait_name|
+  match do |instance|
+    instance.traits.include?(FactoryGirl::Trait.new(trait_name, &@block))
+  end
+
+  chain :with_block do |block|
+    @block = block
+  end
+end