Fixes a compatibility issue with python 2.6.5

Sphinx still keeps compatibility with 2.6.5 so should we.
Add support for headings.

Headlines are rendered as emphasized text as it has been done for
doxygen sections. Section levels are considered because the doxygen XML
structure does not comply very well with the docutils toc tree. This
would require to transform the whole XML tree, though.
Format docstrings in pep-8 style
Provide headings example based on rweickelt's code

It has been extended in include a <b> tag within one of the headings as
doxygen seems to allow that so we might as well test it.
Extend docHeadingType build & rendering code

To take into accout multiple children of varying types. We probably
should allow loads of different node types as children as the .xsd file
seems to suggest (perhaps as many as the docParaType does) but for the
moment we stick with the basic inline styles.

We then change the render code to iterate over the contents array and
render each entry before wrapping them all in an emphasis tag. I
previously did not really the emphasis tag could wrap around other
elements rather than just text but it is apparently allowed.
Add difference document to help pages

To highlight differences between the Breathe output and the Doxygen &
Autodoc outputs which people might be expecting.
Merge branch 'doxygen-heading-support'
Fix missing entry from examples Makefile

In the refactor I must have lost it somehow.
Add basic production & development requirements files

I'm not sure exactly how best to handle these but it seems like a good
step forward to have them somewhere.
Switch change log entry to Added

As people who want to use it will have to specify the future breathe
3.1.0. If we leave it as a bug fix then they'd have to specify 3.0.1
which isn't the point of patches.
Add changelog entry for headings support

Thanks to rweickelt for this.
Merge branch 'fix'
Move to v3.1.0