Replaced static doc directory with wiki submodule

This deletes the doc directory from the root and adds a git submodule
pointing at the titan wiki git repository.  After checking out, the
contents of doc can be pulled from the wiki git repository by the
standard git submodule commands:

git submodule init
git submodule update
Adding experimental titan-site module

Exploring whether our gh-pages can be automatically generated using
"mvn site" build phase
Tweaking doc submodule and titan-site
Tweak titan-site pom.xml
Hacking on titan-site

gollum-site for wikidoc export, javadoc generation, gh-pages upload,
and s3 upload are all working

As I've experimented with the configuration in titan-site, I've begun
to think that it could be pushed into the top-level pom and the
existing child poms, at which point titan-site would no longer need to
exist.