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
Don't try to load custom file if it doesn't exist (fixes #65)
For now, skip erlang support if pkg is uninstallable

See #63
Enable spell-checking by default (needs ispell or aspell installed)
Tidier workaround for uninstallable Erlang package
Don't require ido-complete-space-or-hyphen
Add tagedit
Add git-gutter-fringe
Add imenu as a default auto-complete source
Move 'require-package calls next to the related setup code
Fix hl-sexp anti-flicker advice
Better docstring for require-package