Created tests indicating behavior

- Identifier name defaults to "id"
- Identifier name is mutable
- getIdentifier() uses identifier name to lookup identifier
Implement identifier name property

- Added $identifierName property
- Added mutator and accessor for $identifierName
- Modified getIdentifier() to use the $identifierName when attempting to
  retrieve the resource ID.
Merge branch 'feature/restfulcontroller-configurable-identifier' of git://github.com/weierophinney/zf2 into feature/4181

Close #4181
Merge branch 'feature/4181' into develop

Close #4181