Add transformer between keys and objects in dicts.

- Can for example be used for transforming between enum values and
strings.
Add tests for key/objects value transformer.
Change enum description to be lower case.

- In the test description that is.
Fix stylistic issues.
Assert that the dictionary is not nil.
Move value mapping transformer test enum into global.
Remove __block storage modifier.
Fix using NSUUID which required OS X 10.8.

- Mantle’s deployment target is 10.7.
Rename the value mapping transformer to just that.
Assert that the dict for value mapping is bijective.
Enumerate instead of allocing an array.
Merge pull request #135 from rastersize/enum-dict-transform

Add transformer for transforming between the keys and objects of a dictionary