Doc corrections

Mostly spelling corrections, and removal of the notice that H.264 is the
only supported video recording format
Fix #21

Added resizing capability to video recording and all capture methods,
updated documentation and tests accordingly
Fixed bug in raw_format getter

The raw_format getter was querying the camera's video port which is now
permanently set to YUV output
Fixed docs for frametrate and resolution

The docs incorrectly claimed a limit of 15fps above 1080p, but 15fps is
only enforced at the maximum resolution of 2592x1944
Fix #32

Changed raw captures to have format specified directly on each capture
method, and used the resizer's output format to handle RGB conversion.
Unfortunately RGB encoding isn't accepted by the resizer, but RGBA is so
some manual code is added to the raw encoder class to handle stripping
the unused alpha bytes.

As the raw encoder stuff is common to both the one and multi image
encoders, and as it's now rather more complicated, it also made sense to
consolidate the code in a mixin class.
Use finite timeouts where possible

After a couple of hangs in testing, it makes sense to impose a 30 second
limit on individual captures (similar to the timeout for split_recording).
xfail a couple of raw tests

For some reason the camera seems to hang on them...
xfail some raw tests

No idea yet exactly why they fail. I thought it was because the resizer
doesn't like 100x100 as a resolution, but it works fine with cooked
output (JPEG et al). 128x112 works (aligns with rounding done by the
camera internally), but so does 128x128 (which doesn't), while 112x112
doesn't work (which rules out there being some other rounding that the
resizer's working to)... Yeah, I'm mystified (for the time being!)