For #12, add libraspberrypi0 to depends

As per Alex's suggestion, libraspberrypi0 contains libmmal which is a
dependency of the library
Fix #36: return namedtuple for frame property

The frame property no longer returns a simple integer frame number, but
instead returns a PiVideoFrame namedtuple which provides a variety of
information about the current frame and the state of the video as of the
current frame (index, frame_size, video_size, etc.)
Use a smaller buffer size for file writing

Smooths out writes when recording video (1Mb results in "bursty" writing
which can cause issues on slower storage)