Misc enhancements @Colliotv suggested

[crccheck]

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
• [x] add changelog to project

[crccheck]

bleh, waited too long. going to merge as is.

[efe123]

nice