First stab at getting rapid image capture working

Using the method proposed by MacHack on the RPi forums of using a JPEG
image encoder attached to the camera's video port
More debugging

More prints, a change to the port's encoding (OPAQUE to I420),
frame-rate, and so forth ... no idea why it's hanging the camera yet
Doh, wrong superclass
Ensure previewing doesn't raise an exception

If an error occured during camera init, previewing could throw an
exception during the subsequent clean-up
Make ISO 0 by default

This is the "auto" setting and allows exposure_mode to work which should
cut down on some confusion for new users
Common superclass for image encoders

Replaced _PiStillEncoder and _PiSequenceEncoder with a common
superclass: _PiImageEncoder which has _PiOneImageEncoder (for capture
and capture_continuous) and _PiMultiImageEncoder (for capture_sequence).
Tweaked the encoders so that they're the ones that do the test for
closed cameras and pre-existing encoders - no point duplicating the code
in all the different capture methods.
Fixed framerate and added property to adjust it

Fixed default framerate back to 30 and added a new property (framerate)
to modify and query it. Updated resolution to take account of custom
framerate when reconfiguring camera.
Merge branch 'iss2'
Updated tests to use non-deprecated name
Bumped revision for release and updated trove classifiers