add support for drain

[polomsky]

Tested on Mac OS, Linux and Windows with UART emulated on USB.

I wrote general BSD code, because it should work, but I am not able to test it.

On Mac OS I am quite sure that drain behaves as needed. For Windows it seems that drain is actually part of Write, because Write was slow and Drain was fast. But I have not found any mention in Win32 docs about buffering and asynchronous write, so I kept it. For Linux Drain is also taking more time than writing.

[wissamalkja]

great ,,

[smuroff]

@wissamalkja could you merge it? I've looked for the same feature.

[cmaglie]

The Linux implementation has been replaced with #159.