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)


Good question. @dwhly and I spent a good amount of time talking about this, and I have identified the following reasons for hiding the username when editing:

1. Space: some of the other options we tried for write preview weren't as visually appealing. We were considering for example, keeping the user name where it is and putting the write / preview controls in the middle, but that didn't look nice.
2. In preparation for a world with multiple annotation stores, and perhaps multiple accounts (w3c, hypothes.is, etc), we can't assume that the username for an annotation will be the hypothes.is username. There is some design work to be done here, but hopefully hiding the username will get us heading towards the right direction. When we work on the privacy control to support groups or even multiple annotation stores I suggest we revisit this question.
3. There is already some indication of the currently logged-in username in the app
![username](https://cloud.githubusercontent.com/assets/521978/4002621/87010932-296e-11e4-80e6-d750940c5f1f.png)
4. We're not the only people to do this: Facebook doesn't have user names next to posts (it's assumed people know that they are logged in):
![facebook](https://cloud.githubusercontent.com/assets/521978/4002672/f9660040-296e-11e4-9fe7-a28a7bb53765.png)

Thoughts?