B-tree implementation functionally complete, tests written, and Doxygen comments written in jnxbtree.h

I've also had to modify Doxyfile slightly to exclude *.c files. I had to enable MAN_LINKS in order to be able to search for function names using man, but that flag pulls in all the function definitions, even the ones from *c file, which I don't want users to see.

Our API is defined in .h files anyway so I couldn't think of a reason why I wouldn't exclude *.c files.