Add style guide rules about modules and requires

@spicyj @benkomalo @chrisklaiber 
I tend to think that it looks a little better with a blank line before the lowercase imports, but I don't feel super strongly. Can we have an example of pulling out individual exports, something like this?

```
var React = require("react");
var ReactART = require("react-art");
var _ = require("underscore");

var Group = ReactART.Group;
var Path = ReactART.Path;
var Shape = ReactART.Shape;
```

We do this occasionally for first-party code too (I think) but it's rarer.