Disable PNG tests

PNG generation fails under latest firmware; disabling until upstream can
be fixed
Update picamera to use new modes for #65

The most recent firmware update introduced a whole bunch of new camera
modes (thanks guys!), including 60fps and 90fps modes. This commit
changes the framerate property to permit access to these modes, and
tweaks the tests to support the changes. It also changes the shutter
speed tests to work as shutter speed is now (correctly) clamped to the
current framerate (i.e. if framerate is 30 you can't set a shutter speed
slower than 33ms).
Fix #64

Re-wrote the stills vs preview resolution to cover the new modes
introduced in firmware 656 - the section reads a bit more linearly now
too. All references have been fixed up, and the recording full res
recipe has been removed as it's now effectively redundant (most
resolutions below 1080p should record full FOV just fine).