Replace STKeychain with SSKeychain

This Pull Request replaces the STKeychain with SSKeychain (https://github.com/soffes/sskeychain).

Issue #202 was still bugging me sometimes, until I found this issue for SSKeychain: https://github.com/soffes/sskeychain/issues/75

By adding ```[SSKeychain setAccessibilityType:kSecAttrAccessibleAlways];``` this should always allow Keychain access in background. This should also fix: #262 

Kind regards,

Patrick