Add a USB device info viewer than demos the USB device picker.

This app displays information about any device available to Chrome. The
new chrome.usb.getUserSelectedDevices API is used to request permission
for arbitrary devices.
Move the USB device info app under the samples directory and add README.
Updated README.md to include screenshot and added icon.

Also fixed some small nits.
Merge pull request #330 from reillyeon/device-info

Add a USB device info viewer than demos the USB device picker.