Adding new user and user/repo shortcutst to Go form.
Starting to improve the shortcuts docs.
Main page layout and shortcut table.
Cleaning up shortcut table.
NbViewer
Addressing review comments:

* Center section headings.
* Use lowercase nbviewer for branding
* Eliminate shortcut table and replace by placeholder
* Adjust vertical spacing.
Remove extra hr on home page. There were 3 at the page bottom :(
Merge pull request #254 from ellisonbg/extrahr

Remove extra hr on home page. There were 3 at the page bottom :(
Fixing footer formatting that got broken in recent work.
Merge pull request #239 from ellisonbg/goform

Clean up layout of main page and add shortcut table
Merge pull request #255 from ellisonbg/footerfix

Fixing footer formatting that got broken in recent work