Add markdown preview, add markdown help icon, and adjust styles for markdown output

This PR addresses the following issues: https://github.com/hypothesis/h/issues/826, https://github.com/hypothesis/h/issues/1262, and https://github.com/hypothesis/h/issues/1354

It adds markdown preview and a markdown help icon:
![currentinterface](https://cloud.githubusercontent.com/assets/521978/3974681/694ae362-27f8-11e4-8657-de213bb937ae.png)

I also changed the Placeholder text to add some examples since I wasn't sure whether the icon alone was enough. Need to usertest with users who have never heard of Markdown. 

The PR also improves the markdown output stylings for annotation bodies:
## Before:
### Collapsed:
![beforecollapsed](https://cloud.githubusercontent.com/assets/521978/3974747/b42cdab0-27f9-11e4-8dec-53218c87da08.png)
### Expanded:
![beforeexpanded](https://cloud.githubusercontent.com/assets/521978/3974746/b19ca582-27f9-11e4-84c6-0fb66a60a8d8.png)

## After:
### Collapsed:
![improvedcollapsed](https://cloud.githubusercontent.com/assets/521978/3974686/8e7b0310-27f8-11e4-9ba4-790f15b2ffa6.png)
### Expanded:
![improvedexpanded](https://cloud.githubusercontent.com/assets/521978/3974689/a65cfc86-27f8-11e4-99b7-f02121678495.png)


Feels like we should maybe move the usernames above the excerpts always.