portaudio: C++11'ish the code

Use many C++11 stuffs to make the code clearer and well strutured:

* pimpl idiom to remove the portaudio.h dependency from header file
* array and for-range
* algorithm

(And various little changes for coding rules compliance)

Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>
Change-Id: Iaa11e16aecb1bd36200c820e4cb1aa657479eb60