Return to bare gist functionality

ESC key and pressing the bookmarklet again should return you to the raw
gist. Fixes #13.
style refactor; cache slides selector in $slides, tighten up some comments and whitespace
Merge remote-tracking branch 'upstream/master' into return_to_bare_gist

Conflicts:
	gistdeck.js
Merge remote-tracking branch 'vitorbal/return_to_bare_gist'

Conflicts:
	gistdeck.js
reset slide margins too