clean up TranslateableProperties spec

No reason to have two describe blocks.
eachTranslatedAttribute gracefully handles null translation keys

If an object has a `___Translation` key that points to a null value,
`eachTranslatedAttribute` will call the callback with `null` instead
of trying (and failing) to get a translation.

Closes #150
Merge pull request #167 from jamesarosen/james/null-translateable-property

eachTranslatedAttribute gracefully handles null translation keys