Allow for region override in search API (bug 908519)

Here's a preliminary patch for [bug 908519](https://bugzilla.mozilla.org/show_bug.cgi?id=908519).

The problem: we want to allow clients to bypass region filtering in app search results without bypassing region detection. This is necessary for rocketfuel, so curators can add apps to collections that are typically excluded from the region where the curators are working.

The solution: allow `None` to be passed as a value of the `region` querystring param, which bypasses region filtering only in the search API.

Problem with the solution: this implementation will allow consumers to bypass region restrictions. This could be problematic with some countries that legally-restrict access to apps.

r? @robhudson @mattbasta

Thoughts on the problem, @clouserw?