Fix encoding issues with certain urls, verify that it uses the actual NSURL item, and updated the activity title.

- String encoding now uses encodeByAddingPercentEscapes() instead of stringByAddingPercentEscapesUsingEncoding: because specific URLs (weather.gov) were not being encoded properly.
- Cleaned up the prepareWithActivityItems: code so that it gets the actual NSURL activityItem instead of the last item. This fixes [issue #8](https://github.com/alextrob/ARChromeActivity/issues/8).
- Changed activity title from just "Chrome" to "Open in Chrome" this makes it match [TUSafariActivity](https://github.com/davbeck/TUSafariActivity).