* Implement oauth secret token in headful-lite. This is useful for Twitter sign in.
* Deprecate TwitterAuth{Provider,Credential}. OAuth sign in flow is now the recommended way to sign in with Twitter.
* Add update OAuth credential to AuthDataResult. Developer can access the secret token from that.
* Replacing FIR_SWIFT_NAME macro with NS_SWIFT_NAME.
This pushes the minimum Xcode version to 7.3, as NS_SWIFT_NAME was
limited before that version (which is why the macro was introduced in
the first place).
* Fixed FIRMessaging header