Allow user to specify link display text by subclassing `UriCell`

See wyuenho/backgrid#41

This PR allows users to control the display text of a link formatted by `UriCell` by overriding a function `displayText`. The existing solution is to write a custom cell class, which requires duplication of logic in the render method; with this PR the operation is more straightforward and does not duplicate any logic in the render method.

The PR includes one new test:

![image](https://f.cloud.github.com/assets/52205/306100/2cc2a186-966e-11e2-85f7-20cc14fd2903.png)