Add code styling to clarify inline code snippets and fix readme formatting

https://bugzilla.mozilla.org/show_bug.cgi?id=1034201

Currently inline code snippets, using the `<code>` tag are rendered like this

![current style](http://i.imgur.com/RgxqfgI.png)

This styling makes it difficult to differentiate between the code and the surrounding text as the only differentiator is the change from a variable width font to a fixed width font.

Github uses, in my opinion, an excellent styling to call out inline code by
* surrounding the span in a box
* using a different background

This PR adds a `<code>` style css block which creates a clearer styling. With this style in place, inline code now looks like this :

![proposed style](http://i.imgur.com/eNbC4ur.png)