Add option to set emulated asynchronous mode to handlers.

The need for this feature arises when asynchronous mode is used while postRunnable() and sendMessage() have been overloaded (to circumvent the Handler altogether by calling runnables on current thread.) Now, when more requests are further dispatched from the runnables (their thread belond to thread pool), AsyncHttpClient raises an exception that current thread hasn't called Looper.prepare().

On the same subject: Why exactly do we need this exception anyway? Why not just log the situation as a warning, remove get/set-UseSynchronousMode() altogether and then this merge request can also be dumped.