Access option for RSA keypair in Keychain

[henrinormak]

Slightly refactored PR from #43. @colinmorelli, if possible, please review this. My main concern is the fact that the access flag can not be changed once set without regenerating the key. If this is a problem, then it needs to be addressed (for example, someone wanting to change the access as part of some switch in the app, without revoking the keypair).