Added argument/ability to set the frame interval (fps)

When using the PS3Eye on the BeagleBone Black, a resolution of 640x480
at 30 fps causes the select() call in v4l2 to timeout.  This seems to be
caused by the amount of data that is transmitted over the USB; however,
capturing at 15 fps succeeds.
Merge pull request #4 from lemoneer/master

Added argument/ability to set the frame interval (fps)