Remove Element.h import

This import created an import cycle between Attr and Element and caused a forward declaration error in SVGKimage.h
Merge pull request #224 from samskiter/2.x

Remove Element.h import in Attr.h
Improve stringFromSource to use the NSData+NSInputStream category.
Merge pull request #226 from dgileadi/2.x

Improve stringFromSource to use the NSData+NSInputStream category.
Merge branch '2.x' of https://github.com/SVGKit/SVGKit

Conflicts:
	Source/Parsers/SVGKParser.m