Added markers to histogram

This allows you to specify a query that will be used to add markers to a histogram. The tooltip field and sort order for picking the top N events are configurable

![screen shot 2013-10-30 at 10 24 15 am](https://f.cloud.github.com/assets/1250387/1439661/2026e0ba-418e-11e3-8497-e8897dc4a254.png)

![screen shot 2013-10-30 at 9 52 26 am](https://f.cloud.github.com/assets/1250387/1439664/2cf36476-418e-11e3-9b2c-d2f91c67e16e.png)


Closes #45 and #350.