Misc enhancements @Colliotv suggested

Based on @Colliotv's suggestions:

* Adds ability to interpret `style` attribute. I found that if you have an attr and style, style wins. So I moved the logic to the bottom.
* Adds a callback option (will remain undocumented for now). I put the callback in a more generic `options` object argument, and skipped passing in node id and attr because you can derive those from the original svg element that I pass in instead. I'd rather not force you to return a raphael object, but that seemed like the simplest way to get filtering to work.

I didn't get around to the xlink:href because I'm too tired to verify it right now.

Closes #14 

#### TODO
* [ ] handle `xlink:href` attribute
* [ ] add changelog to project