Renamed Url/url to URL

As Cocoa uses "URL" (all caps) in places like `+[NSBundle URLForResource:...` etc, I've renamed the instances here to use uppercase for consistency.

I've bumped the podspec to version 2.0.0 as it's a breaking API change also :)

While we're at it, any thoughts on prefixing/namespacing the methods? As `-URLEncode` could be implemented in other frameworks.
D'oh, typo. Good point. Fixed in cd07a75.