First stab at implementing #15

Surprisingly little code needs to change to build a raw encoder; some
serendipitous separation of methods earlier!
Makefile should use default python

If that happens to be 2, don't force it to be 3. It'll be 3 in a py3
venv anyway...
Merge branch 'master' into iss15

Pull Makefile change in
_set_led not working

GPIO was being treated as a local as it was set without a corresponding
global statement
Merge branch 'master' into iss15
Move Exif stuff to PiOneImageEncoder

The only time Exif tags are ever produced is with the still image port
with a JPEG encoder. Therefore, the Exif handling methods should be in
PiOneImageEncoder instead of the base class
More refinement for #15

The PiOneImageEncoder class now contains only the methods required to
output a single image. All the Exif stuff is now moved to a "cooked"
class. The "raw" class now inherits the one-image stuff from its new
base class. The PiCamera class has been updated so that all capture
methods support raw format
Missed an import
Changed 'yuv' format to 'raw'

Also added raw_format property to configure the format of the ports of
the camera. Need to test whether we can change this without disabling
the camera...
Camera needs disabling to change raw format

Also tweaked MMAL module - doesn't need to return a strict ctypes
uint32 for FOURCC's - the integer value will do. This allows the value
to be hashed (for use as a dict key)
Removed unsupported/broken raw formats

Cut the raw_format parameter down to yuv (I420) and rgb (24-bit RGB) as
those are probably the only modes people are interested in. Added a note
to the docs explaining that other formats are possible
Added tests for raw capture
Added py2 compat stuff to preview tests
Added Py2 compat stuff to test config
Removed redundant stuff in setters

The resolution and framerate setters had several redundant lines left
over from copying stuff from __init__
Some unfinished work on tests...
Refined tests

Separated previewing concept out of the camera fixture and refined the
tests accordingly. Added tests for raw capture, and removed tests for
GIF and BMP capture as they tend to crash the camera (hard) when run at
full resolution (2592x1944)
Fixed continuous

When deprecating a generator, the deprecator must yield the new
generator...