Improve stringFromSource to use the NSData+NSInputStream category.

When I added NSData+NSInputStream Adam mentioned that he thought he'd seen similar code somewhere else. I couldn't find it at the time, but of course it was something that I'd done. This pull request fixes the other code (`stringFromSource:` in SVGKParser) to use NSData+NSInputStream. Besides being DRY the NSData+NSInputStream code is safer; the old code in `stringFromSource` had the possibility of generating invalid strings if reading the source data happened to split a multi-byte character since it used string appending.