ddox documentation generator

[Sönke Ludwig]

Okay the more or less final incarnation now includes full text symbol
detection with complete cross linking. I just had to modify some macros
in std.ddoc and algorithm.d to avoid that they produce their own links
and the result is this:

http://vibed.org/temp/phobos/std/algorithm.html
http://vibed.org/temp/phobos/std/datetime.html

There are still some glitches for various reasons and no links are
generated in code blocks right now. But all this linking requires
absolutely no manual linking or use of macros anymore (except escaping
identifiers that should not be linked with an underscore)