Use specific font stacks for grids and grid units

By setting _specific_ font stacks grids, it can be assured that the letter and word spacing will be applied correctly. The font stack for grid units is then set back to `sans-serif`, which is also the default font stack that Normalize.css applied to the `<body>`.

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This is an alternate approach to dealing with issue #41. This approach means that Grids will work out of the box on the most number of OS/browser combinations possible, including Firefox on Ubuntu. The burden is then placed on developers who want to use a custom, non default, font stack in their web projects.

By default, Normalize.css sets the `<body>` to `font-family: sans-serif;`. Normally when developers want to use a different font they simply add a new style rule for `body` switching its `font-family`. With this proposed change, developers would also need to target their custom font stacks to Pure's grid units, otherwise content inside of Grids will be in sans-serif.

The following example shows how custom font stacks would be set if this change was merged in:

```css
body,
.pure-g [class *= "pure-u"],
.pure-g-r [class *= "pure-u"] {
    font-family: Georgia, serif;
}
```

**Note:** The above code and related documentation for using custom fonts with Pure Grids would be highlighted on Pure's website.