update with the authors latest changes

This update includes the latest changes from the authors downloaded from
http://omattos.com/pifm.tar.gz
with the README.md file copied loosely from
http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter
modified mainly for MarkDown and attribution.
fix open calls

Carries fix to open() calls submitted by bmwiedemann in commit 545b2fc.
add GPL boilerplate to source, COPYING file

It was not clear at first submission what license the authors had granted.
However, the wiki says clearly that code is GPL.
Merge pull request #4 from petiepooo/dmaUpdate

Latest code updates from original authors, GPL boilerplate and COPYING file