UIActivityViewController support for iOS6+

By setting the new boolean useActivityViewController to YES, SVWebViewController will now present a UIActivityViewController instead of a UIActionSheet. SVWebViewController also now has an applicationActivities property where the programmer can pass in an array of custom UIActivities to display alongside the included ones.

For this implementation, I have included git submodules [TUSafariActivity](https://github.com/davbeck/TUSafariActivity) and [ARChromeActivity](https://github.com/alextrob/ARChromeActivity) for SVWebViewControllerAvailableActionsOpenInSafari and SVWebViewControllerAvailableActionsOpenInChrome because UIActivityViewController does not come with these activities. However, this means any person who clones your repo has to now do the extra step of updating the modules, so if you were aiming to be the easiest WebViewController to add to a product you may want to remove references to these.