pages: fixes 

(backported from a theme I am currently working on)

- add headings and `.panel-default` class to panels
- add a README panel with the rendered content from `README.md`
- `grunt watch` also builds the pages

Note that the pages are still in flux while I figure out which `package`-/`bower.json` properties we can use 
and what we should add to the 'themestrap' property. Example: "guessing" the correct location for `bower install` if it is *not* published does not work, so we might need a 'themestrap.pkg_url' property.