Use the new chrome.hid add/remove events to support multiple blink(1)s.

This change allows the user to have multiple blink(1) devices connected
to their computer. A selection box is used to switch between them. The
new chrome.hid.onDeviceAdded and chrome.hid.onDeviceRemoved events are
used to keep this list up to date.
Check for existance of HID add/remove events.

Check for whether these events are provided by Chrome before attaching
event listeners so that this code is compatible with older versions of
Chrome.
blink1: Update version number.
Merge pull request #345 from reillyeon/blink1

Use the new chrome.hid add/remove events to support multiple blink(1)s.