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)


Is the `right` value chosen so as to be clear of the scrollbar? If so, that's reason enough for me to say we shoud put it elsewhere, since the scrollbar width is not consistent or predictable and will result in this floating weirdly far from the right edge in some browsers and right up next to it, or even underneath the scroll bar, in others.