use unprefixed Gamepad API if available for WebKit (fixes #23)
Merge pull request #24 from cvan/unprefixedWebKit

use unprefixed Gamepad API if available for WebKit (fixes #23)
Adds mapping for Dualshock 4 controller in Firefox.

Chromium seems to directly support the PS4 Dualshock 4 controller,
but Firefox has some really strange mappings. This adds correct
mappings for the DS4 controller under Firefox.

Tested in Firefox 29 for Linux.