Update README and code styles

:fork_and_knife: 

I found the styles in READMEs a little inconsistent and difficult to read. I updated:

- the style of `code` and `pre` elements in the app to be a little less different than the rest of the text
- the margin bottom of headers in READMEs
- removed the 90% font-size declaration for most README text elements

The `code` and `pre` elements now look like:

![screen shot 2014-09-02 at 3 08 14 pm](https://cloud.githubusercontent.com/assets/928367/4124155/1a4ef062-32d6-11e4-8e9c-e92be8563eac.png)
![screen shot 2014-09-02 at 3 08 22 pm](https://cloud.githubusercontent.com/assets/928367/4124156/1a5226a6-32d6-11e4-945d-107dc3c7d199.png)