Require .btn-default for regular buttons

* Instead of , use  for standard, gray button
* Improves ability to customize buttons by encouraging folks to not override .btn and thus overriding all button modifier classes
* Updates docs to reflect the change
* Reorganize buttons CSS
Merge pull request #7809 from twitter/bs3_adding_btn_default

BS3: Require .btn-default for all standard buttons
Update jekyll setup so works for both localhost and github project page.
Move for gh_pages.