added option to use em units for min and max font

This allows for using 'ems' for minFont, maxFont, minimum, and maximum, via a useEm boolean.  fontRatio and lineRatio is essentially untouched  A dirty example of usage would be:

```javascript
$('#em').flowtype({
	minimum: 31.25,
	maximum: 75,
	minFont: 0.75,
	maxFont: 2.5,
	useEm: true
});
```