Remove the drop shadow from the highlight states

This has been bugging me since I started using the app, so I thought I'd offer some alternative styles.

![napkin 24-07-14 11 17 46 am](https://cloud.githubusercontent.com/assets/47144/3686091/86cadbee-1313-11e4-8cc8-a6eb64b3de3c.png)

I'd like to be able to do something with the outline of the highlight but I don't think this is going to be possible until we move to a less invasive highlighting method. As soon as you put shadows, borders or outlines on the spans it breaks up the text in the highlights and looks broken.

@dwhly @RawKStar77 would love your feedback on this.