Enable GIF tests

GIF seems tolerably reliable (doesn't cause lockups) under the latest
firmware so re-enable the tests, except for the still-port based ones at
full resolution (which cause OOM on the GPU). Also tweaked main modules
to use xrange on py2
Workaround #73

This isn't a permanent fix; it just restores the default still port
encoding that picamera used prior to 1.3, which should ensure that users
wanting to peform full-resolution YUV captures can do so. However,
attempting to perform JPEG captures afterwards will cause future
full-res YUV captures to fail until the camera is re-initialized (see
discussion in #73 for more details)
Fix test in init_preview

Test wrongly targetted the preview component instead of the null-sink
Modified scopes so #73 will test correctly

Fixed up the scopes of all fixtures, and removed a few xfails that now
succeed in the capture portion of the test suite. If the upstream issue
for #73 gets resolved, remember to revert the scope of the camera
fixture to ``module`` or even ``session`` so that the camera isn't
re-initialized between tests
Keep up with mmal changes

Looks like there's some new parameters to add to the MMAL header
translation: inline motion vectors (new parameter for start_recording
maybe?) and no image padding (might be useful on raw captures?)
Fix #83 and re-work raw captures

Re-wrote a lot of the code surrounding raw (YUV/RGB/RGBA/etc.) captures
partially to fix #83 and partially to bring picamera much closer to
raspiyuv in behaviour. Resolution values in camera and encoder
configurations are now aligned appropriately (to 32 for width, and 16
for height).

More importantly, picamera only uses a resizer to perform raw
conversions when absolutely necessary (i.e. when video-port based
captures involving the splitter are requested, or when a resize is
explicitly requested). Otherwise, it manipulates the encoding of the
still port and dumps data directly from the port output. This also
solves the long-standing issue with RGB captures being slower than RGBA
(although only for unresized still-port-based captures).

The test suite has been updated to remove the relevant xfails as well.
Update the changelog for the new release