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)


Prefer `@include` over `@extend` wherever you can. Extend changes source order and if used frequently leads to very hard to read CSS with hundreds of selectors per style rule. Gzipping the css generally minimises any overhead of the duplicated code.