Documentation fixes
Use python's warnings

Converted the horrid print statements in the mmal and bcm_host modules
to actual python warnings (more useful for anyone wanting to, for
example, raise an exception in the case of attempting to use the library
on a non-Pi system)
Lots of little doc fixes
Add header and position to frame attribute

The new header property indicates the frame is actually an SPS/PPS
header in the stream. The position property calcualtes the position of
the frame in the stream.
Fix #39

Added a stream implementing a ring-buffer, and a derived stream
which adds keyframe indices to this stream. Docs and tests added.
Added Raspbian install instructions