Fill method

Not yet ready for primetime as of this pull request. Still lacking getter functionality for images. Patterns are also a tricky beast. Currently defining an `<image>` tags properties will also define those of the pattern. So I'm open to input as to what the SVG pattern api should be. Perhaps the pattern itself is an object containing methods for creating it's sub elements? 

For now, `_pattern` is only capable of setting an image.