Highlight bulk edit tools and include guidance

Analytics and user research sessions have shown bulk edit tools could be made clearer. Many users are still using the "Edit mapping" buttons despite this being a slower process. This PR attempts to highlight and explain the bulk edit tools a little more. 

* Give checkboxes more contrast
* Give the mapping column a header that suggests a purpose, along with a tooltip explaining how to use
* Include a tooltip on the disabled buttons to explain how to enable and use them

![screen shot 2014-04-10 at 17 49 47](https://cloud.githubusercontent.com/assets/319055/2670623/3d7b4bee-c0d0-11e3-9c23-93743276da12.png)
![screen shot 2014-04-10 at 17 50 18](https://cloud.githubusercontent.com/assets/319055/2670622/3d7b36cc-c0d0-11e3-86bf-06e1f0e2aebd.png)
![screen shot 2014-04-10 at 17 50 06](https://cloud.githubusercontent.com/assets/319055/2670624/3d7b91bc-c0d0-11e3-869c-ad71b0972f48.png)