Add accessibility type to inidivdual items

I've found that the global `+[SSKeychain accessibilityType]` is insufficient for our needs as we need to set different types for different objects. I *could* change the global setting whenever I want to write a particular item but that feels hacky and error-prone as you'd have to remember to set it back to its previous value when you were finished.

It also made sense to add OS X 10.9 support at the same time.