Added Android specific methods to the callbacks. 

* Have added methods to PlainPost as of now. If it looks good, I'll add them to zero bin too. 
* loginFailure() -- I've taken into account two cases when a login failure can occur. Have added methods for both these cases. 
 * authToken expires -- redirects to Login Screen
 *  no data connection  -- redirects to Home Page
* pendingLogin() -- Shows a dialog "logging you in.." while verifying authToken
* submit() -- shows "creating.." dialog while creating a new post
* postCompleted() -- hides the "creating.." dialog
* pendingPost() -- hides the "logging you in.." dialog