Add "Open in Chrome" activity

Should all work, could do with a double check though.

Had an internal debate about what to do in the case of the user not having the Chrome app installed. At first I had the ```init``` method return nil but that leads to a crash if the developer doesn't check for nilness and tries to make an NSArray with nil in it. They could easily miss this unless they test by uninstalling Chrome and trying it out. So instead I opted for an "install from App Store prompt". Not ideal, but not a crash. Could always put a snippet in the  README demonstrating how to check if Chrome is installed, and if it isn't don't create the activity?

Also considering adding support for the ```x-callback-url``` scheme (https://developers.google.com/chrome/mobile/docs/ios-links) which I'll do with a separate ```initWithBlah:``` method.