Small CSS Adjustments

Noticed a few things when I was looking over some of @aron's stuff. Two small regressions since we implemented the pattern library:

1. These icons were positioned poorly within the buttons:
![hypothesis2](https://cloud.githubusercontent.com/assets/521978/3688504/0b5b5ce4-1337-11e4-8e8c-7baa6fd577fd.png)

2. The margin bottom for the form inputs was a little too large in my opinion:
![killmarginbottom](https://cloud.githubusercontent.com/assets/521978/3688524/27e579ee-1337-11e4-9741-d5a2319670c5.png)

3. Tags were ugly:
![hypothesis3](https://cloud.githubusercontent.com/assets/521978/3688562/8b158702-1337-11e4-9874-36bc7b6b9338.png)

4. I decided that the "Markdown is supported." should move to a placeholder inside the text area. Rational: I think it's ugly.
![killmarkdown](https://cloud.githubusercontent.com/assets/521978/3688579/ad500e00-1337-11e4-9736-4486d853c7f5.png)

So this PR fixes spacing between icons and text inside buttons, improves spacing on annotation cards, adds placeholder for annotation creation textarea, and adjusts padding on tags.

Here are screen shots of the changes:
1. This shows the fixed icons and the new textarea placeholder:
![placeholdermarkdownmessage](https://cloud.githubusercontent.com/assets/521978/3688617/1fb0270a-1338-11e4-9e59-17e81e070623.png)

2. This shows the tags (I also made them a bit smaller), as well as the tighter spacing. 
![smalltags](https://cloud.githubusercontent.com/assets/521978/3688624/2845e576-1338-11e4-958f-cd7e123fa485.png)

Though, perhaps not. In order to provide a markdown reference, we could have a tutorial as a placeholder in the editor; showing both the syntax and what the syntax renders to. Might help teach our users a useful tool.