Differentiate [readonly] inputs from [disabled] inputs

This pull request builds on the one started by https://github.com/yui/pure/pull/102. 

[readonly] form inputs now have a different style from both [disabled] and normal inputs. [readonly] inputs do not have the default `:focus` style either. Here's the difference:

![](http://f.cl.ly/items/3q2L1z2j11102s1E3M1y/Screen%20Shot%202013-07-09%20at%205.48.46%20PM.png)