Updated has(1) and belongs_to specs to be shared
Updated has(1) and belongs_to specs to be shared

* To achieve similar behavior between has(1) and belongs_to it
  was necessary to refactor some of the Relationship classes.
* Updated DataMapper.setup to check for the existence of the
  adapter file, and only load it using an absolute path if it
  exists.
* Added const_added hooks for all adapters
* Removed Property#getter attribute and just made it so that the
  Property#name is used to determin the accessor and mutator name
* Simplified Resource#key to only cache if set
* Updated Resource#_update to reset the cached key

lgierth/dm-yaml-adapter,datamapper/dm-yaml-adapter