belongs_to accept :class_name as well

Extend belongs_to to use same syntax as has_many.
belongs_to setter must also convert to class_name
Move class lookup inside defined functions

Was a mistake to attempt to perform lookup only once, that would require
all :class_name objects to be forward declared.
Code style fixes
Merge branch 'master' into belongs_to_class_name

Conflicts:
	lib/motion-resource/associations.rb
	spec/env.rb