Add simple Doxygen filter for Julia

Also modifies Doxyfile to accept Julia source files, telling doxygen to
parse Julia files with its C parser. Now of course Julia is NOT C at
all, but doxygen allows you to fool it by running input filters on
source files.

The Julia-to-pseudo-C filter is the most brain-dead thing you can
imagine. All the heavy lifting is done by JuliaParser.jl; the filter
simply looks for function declarations, rewrites the signatures into
pseudo-C notation, and then spits out the body of the function parsed by
JuliaParser wrapped with a pair of braces.

The pseudo-C function signature is quite incomplete:
- varargs drop the names of the variable (e.g. myarrays... -> ....),
- kwargs are ignored

Nonetheless it's quite amazing how much functionality you can get from
such a terrible hack!