Add program listing example

As reported by jo3w4rd on #65 on github.
Improve handling when project is not found in config
Provide support for programlisting

We have to parse the programlisting tags within para tags and then
provide renderers for programlisting, highlight and codeline.

We render as a line block instead of literal_block as there can be
references to other parts of the code base which the literal_block
doesn't seem to handle well.
Document custom css usage
Switch to literal block for programing listing

Turns out that if we pass an extra empty string to the literal_block
node at the start then it renders the pending_xref properly. This makes
no sense to me but hey, I've never really grokked this stuff anyway.

Consequently we can remove the CSS stuff, which is good.
Acknowledge jo3w4rd in the README and credits

Thanks for reporting the issue with programlistings.
Merge branch 'programinglisting'
Clarified use of :sections: option in doxygenclass directive.

Found that properties were excluded by default.
Minor reformatting
Merge branch 'jo3w4rd-master'
Fixed potential KeyError in project lookup

As noted in the exception, if breathe_default_project had a value which
wasn't a key in the breathe_projects dictionary then we have a problem.
Merge branch 'quickstart'
Provided basic image example

Based on jo3w4rd's ticket.
Expose Sphinx project source directory to project info

And provide a helper method for getting paths relative to it.
Extract image nodes from the xml and output

We provide the very basics of support for the \image tag in doxygen
output by reading the name out of the xml and outputting a docutils
image node with the appropriate relative path using the mechanism just
committed.

For ticket #67.
Added image example to specific test page

And changed the documentation a little so that it isn't just an out of
context cut and paste of jo3w4rd's example.
Switch to using Sphinx "absolute" paths for images

The code was getting confused when the .rst file that wanted the image
was not at the top level of the Sphinx project.

Here we change to using the Sphinx "absolute" path concept but
prepending "/" to the relative path from the Sphinx top-level folder.
Without the "/" it thinks that the relative path is from the subfolder
containing the .rst file and so gets confused.
Merge branch 'image-support'
Don't needlessly generate HTML

Mistakenly left in there.
Minor coding style clean up

I've left my try-except phase and have realised that there are more
graceful ways to handle those situations.