Refactored Resource#attribute_get and #attribute_set
Refactored Resource#attribute_get and #attribute_set

* These methods now delegate to Property#get and Property#set
  under the hood respectively.  The attribute_* methods knew too much
  about the concerns of Property, and it makes sense to move the code
  to the Property class.  In the future Property will (likely) be refactored
  so that when you define a Property, it uses the type you specify
  to mix-in a module corresponding to that type, which will encapsulate
  all the logic to typecast, defaults, and other type specific logic.
  Routing the attribute getting/setting through the Property get/set
  method allows the per-type modules to alter the behaviour as
  necessary.
* Removed unused lock attribute from Property because the new tracking
  code performs the same function.
* Updated Resource#attribute_get and #attribute_set to be private since
  the only code using these methods should be custom accessors and
  mutators in the Resource.

basex/ardm-do-adapter,datamapper/dm-do-adapter