Merge pull request #90 from parisk/feature/pypi

PyPI Distribution Code
Added new serial class, remove interrupt chaining

The new RawSerial class is a simple wrapper over the serial HAL that can
be safely used from an interrupt handler.
Interrupt chaining code was removed from InterruptIn, Serial and Ticker
because it caused lots of issues with the RTOS. Interrupt chaining is
still possible using the InterruptManager class.
Fix error in 'update_repo' call
Added the new RawSerial.h in mbed.h
Merge branch 'master' into feature/pypi