API doc generator

requires etags binary

very basic for now, only extracting functions decls, and macro and struct names, from c files. It knows how to omit static functions.

I'll move the doc from our adoc files into comments and see how it works

For now it only extracts the only two functions which have a doc:

http://htmlpreview.github.io/?https://gist.githubusercontent.com/mmikulicic/68e50beca4b59738a887/raw/629dc8c227e0d704e83dba2904c6b34bc15c8878/-

TODO: show complete struct definitions and much more.