DocBook reader: Support informalfigure

[pyssling]

Add support for informalfigure.

[jgm]

What about using getFigure here instead of getBlocks?
If the element doesn't contain a title, then it will do much the same thing (except that it will also have the id information available). And when we add a Figure element in the future, we'll presumably want both of these things to produce one.

[pyssling]

done.

[jgm]

Thanks!