Improve navbar UX logic for #233

This change takes us away from using overflow:hidden on body by
default, only toggling this using a new `open` class when the navbar
has been expanded. `open` is toggled on body in the same places as it
is for the appear and navdrawer.
Potential fix for #79

In the 1200px media query, negate the `.app-bar.open, .app-bar.open ~
main transform` to remove the white area (~250px out) displayed. This
generally seems to be a FF only issue.
minifyHtml should only run on html files
Merge pull request #240 from ValentinKlinghammer/master

minifyHtml should only run on html files
Close GH-243: Vendor prefixes. Fixes #175
Merge pull request #239 from google/fix-79

Potential fix for #79
minor consistency tweak for #239
gulpfile - centralize autoprefixer config
gulpfile - simplify error handler
Address feedback on #238
Merge pull request #238 from google/fix-233

Improve navbar UX logic for #233
Merge remote-tracking branch 'upstream/master'

Conflicts:
	gulpfile.js