Major improvements to stability of dragging / dropping and block editing.

other save code to fix #510, but also improve the stability of dragging and dropping and improve the inline add/edit functionality.
Return default site theme if page has an invalid PageTheme
Handle invalid theme return in addBlock method
Add HHVM to travis
finishing up thumbnails interface in dashboard; fixing slight dashboard z-index bug
closes #228
Merge branch 'master' of github.com:concrete5/concrete5-5.7.0
merge
Merge branch 'master' of github.com:concrete5/concrete5-5.7.0
revert
closes #51 reskin group sets dash ui
Merge branch 'master' of github.com:concrete5/concrete5-5.7.0
closes #307 re-skinned, but the db backup is f'd
more work on extendable thumbnail types, listing
Merge branch 'master' of github.com:concrete5/concrete5-5.7.0
Adding in lightbox support to redactor
Update .replace call to use the refactored style so that design saving works
fixes #576