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

Currently a user needs to write a custom Cell class in order to format
URLs with an alternate display text. This requires implementing the
render method and thus duplicating logic in `UriCell`. This commit
allows display text to be specified by subclassing `UriCell` and
implementing a single function `displayText`. E.g. in coffeescript:

class MyUriCell extends Backgrid.UriCell
    displayText: -> @model.get('my_field_name')
Merge branch 'master' into uri-cell-display-text

Conflicts:
	lib/backgrid.js
Update test of UriCell with custom display text

The test function was originally created by copying the test function
for UriCell. This commit makes changes to the custom-display-link test
that were subsequently made to the vanilla UriCell test.
Merge pull request #88 from counsyl/master

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