Switch to ARC

I made the necessary changes to switch to automatic reference counting (aka. ARC). It's mostly removing code so I wouldn't say it's a risky change. In the way I fixed a few bugs, besides all the potential memory leaks:

* Some setters where not correctly handling the case where you reassign the same object. For instance `CSSValueList` `setCssText:`. If you do list.cssText = list.cssText the property would have been released creating a dangling reference.
* In `SVGKParser.m` `errorEncounteredSAX` a c string was being casted to an NSString directly probably causing ugly memory problems.

Fixes #20