Use regular expression-based URL detection, prevent label text from overlapping with custom text view

- URL detection is based on a regex provided by John Gruber (found [here](http://daringfireball.net/2010/07/improved_regex_for_matching_urls)) - this allows for much greater flexibility in URL detection (such as links without the http(s):// schema attached) (https://github.com/wakachamo/STTweetLabel/commit/ed4f988704e44f3b7e18e2dd54cde14e02ec08a1 and https://github.com/wakachamo/STTweetLabel/commit/424c6c411ffdd91185e6686a692d00432c4e71dd)
- Whenever the text is set, the superclass method is called in order to eliminate an overlap with the UILabel class's drawing and STTweetLabel's textView drawing. (e.g. this solves a minor nitpick where STTweetLabel would show overlapping text if it were being used inside a nib as a subclass of UILabel with some preset text defined in the nib) (https://github.com/wakachamo/STTweetLabel/commit/424c6c411ffdd91185e6686a692d00432c4e71dd)

The only implication is the fact that the ```validProtocols``` array is no longer being considered by the URL detection method.

Thanks for reading and also for making such a great UI addition—it's been a massive timesaver on one of the projects I've been assigned, and I wish I'd come across it much earlier in the past! :)