Added mbed-events library ...

Added mbed-events from https://github.com/ARMMbed/mbed-events. Changes
from upstream:

- the whole code is licensed under the Apache license. Sources and
  headers were updates with this information.
- removed the porting layers for Windows and FreeRTOS and the references
  to these porting layers in equeue_platform.h.
- moved the TESTS directory in mbed-events to the TESTS directory of
  mbed-os.