Move gh-pages branch into doc/ of develop branch

as part of #95.  Will send another PR for gh-pages.

The history looks weird on GitHub because I used a [git subtree merging](http://git-scm.com/book/en/Git-Tools-Subtree-Merging) to transplant the tree of 06e3245f067e998a8ac0a5e34218a1bd7bf19d77 (in gh-pages) into docs/.

`make deploy-github` under doc/ will overwrite our gh-pages branch with the compiled website content and push to github.