change the sign in flow to use the same system as the iOS application

[joshlong]

We've used resource password grant for iOS. this basically means we can skip the OAuth flow dance on the server and simply transmit a user and password.

There are numerous reasons about why doing this is no more (or less) secure than using more OAuth-y flows as we are currently doing in the Android module, but for our purposes its also a dramatic reworking of the code.