Disable pointback when using skeleton-insert

Fixes #60.
Wrap inferior-js keybindings up in a small minor mode
Clarify error message when "make relaxng" couldn't be run
Drop the html5 relaxng schemas

These did not result in reliable validation of html5 files, and led to
extra installation complexity.
Don't fset html-mode to nxml-mode
Tidy up the readme
Only enable flyspell if ispell-program-name exists
Tie flyspell-prog-mode to prog-mode in Emacs >= 24
Enable js2 imenu
Merge branch 'master' of git://github.com/purcell/emacs.d into dev