handle v4l devices whihc don't support setting the framerate
some v4l drivers (bttv) only work when the mmap call is for both read and write.
print a warning if the v4l can't use as many buffers as we asked for
Merge pull request #10 from jsarrett/fps_fix

Improvements to framerate control, mmapping and buffer size notification, all to better support non-webcam/UVC devices.