Add rs-link class

In many applications, it's desirable to have a button that's styled like a link. Canon allows this via the rs-btn-link class, but that classes is tailored for buttons and aligns with other buttons. Links within text or tables that use the rs-btn-link class will be subtly misaligned. This pull request adds a class called rs-link, which styles elements to appear like links, minus the alignment associated with rs-btn-link.
Webkit adds a gray ring around focused buttons. Do we want to keep that? Pic below:
![focused-button](https://cloud.githubusercontent.com/assets/805335/5269623/5f82eb36-7a32-11e4-8019-ff53cbebad04.png)