WIP: Add the AsyncClient.

TODO:
1. Add async method on the bridge.
2. Fix get_attribute_from_server for AsyncClient.
3. Add an example to test async.
WIP: Clean up the server to allow threaded request only for instance method calling.
Fix the test for threaded method call.

Apparently, the future object only needs tuples for args and not lists.
Fixed that by type casting to tuple.