Add pagination to search

https://www.pivotaltracker.com/story/show/66589580

These changes add pagination to unified search. Frontend already supports making search queries to rummager with the `start` and `count` parameters, so this pull request goes the rest of the way in adding 'next' and 'previous' links to the search results page.

The pagination links follow the Whitehall pattern. To unblock this story for the time being, I've copied the pagination helper from Whitehall here. In the future, we might want to extract this into the frontend toolkit.

Here's how they look:
![screen shot 2014-08-11 at 15 55 02](https://cloud.githubusercontent.com/assets/71922/3877616/81ec6d1a-2167-11e4-9946-712273befe59.png)

As the pagination links are rendered in the mustache template, they update automatically when filters are applied. However, at the current time, the pagination links themselves don't load previous and next pages inline using Ajax, as this is out of scope for the current story.
Should we be validating that these two parameters are >= 0, rather than testing each time?  It doesn't make sense for either of them to be negative.