Showing registration status for boot camps on index page
* Merging Trevor King's modifications to boot camp compilation to show open/restricted status.
* Adding optional metadata 'registration' to boot camp pages to show if they are open or restricted.  If metadata not present, boot camp registration hasn't started yet.
* Changing format of boot camp index page accordingly.
Merge branch 'master' of github.com:swcarpentry/website into registration
Fixing build complaints in new boot camp index page