Add simple implementation of AccountManager.addAccount()

* Returns correct bundle values depending on presence of activity parameter
* Throws AuthenticatorException if no authenticator matches accountType parameter

As defined here: http://developer.android.com/reference/android/accounts/AccountManager.html#addAccount(java.lang.String, java.lang.String, java.lang.String[], android.os.Bundle, android.app.Activity, android.accounts.AccountManagerCallback<android.os.Bundle>, android.os.Handler)

TODO: Should check permissions for http://developer.android.com/reference/android/Manifest.permission.html#MANAGE_ACCOUNTS