Add basic keychain support

This PR adds a basic "keychain" route that allows some simple interrogation and setting of keychain values. The majority of the Core Foundation keychain API is not exposed - only what is wrapped up by [SSKeychain](https://github.com/soffes/sskeychain). I pulled in SSKeychain (MIT-licensed) and renamed the prefix to "LPSS" - is that kosher, or do I need to reimplement the keychain access using the CF API?

PR for the ruby side: calabash/calabash-ios#323