Promote WebViewApp to first-class mobile web testing platform

One of the current issues with ios web testing in ios7 is that we can't use Safari without going through a launcher. And then, going through the launcher, or testing on a real device, we're unable to handle alerts.

This pull request updates the existing WebViewApp to be a little more full-featured. It then provides the ability to automate this app just as if it were safari, by passing in 'iwebview' as the `app` desired cap. Since we can automate WebViewApp natively, we can also handle alerts.

Multiple windows are not yet supported in WebViewApp.

This pull request also refactors ios.js/ios-hybrid.js and adds a safari.js file which is used for Safari and WebViewApp, helping to keep our code a little better organized.

@snevesbarros, would love any thoughts on this.