Remove commented xfails
Fix #100
Fix up errors found in testing
Fix #106
Disable PNG tests

Occasional camera locks when producing PNGs from the video port. Issue
is known upstream (see discussion in raspberrypi/userland#177) so
disable tests until it can be investigated
Add the debug_buffer routine to encoders

Now we're supporting custom encoders other people might find it handy
for debugging too...
Merge branch 'master' of spider:/home/dave/picamera
Minor doc fixes
Fix #107

Added new picamera.array module, added numpy to package deps on Debian,
and as optional requirement for PyPI package (when "array" option is
specified - added notes to installation instructions). Updated advanced
recipes to reference new custom output classes.

Replaced the mocks in mmal and bcm_host with a mock specifically meant
for docs in conf.py; had to do some work to mock out numpy anyway as RTD
won't support C-based extensions for docs (understandably).