Load search fields from xml instead of building widgets in PHP.

Test case of how we could structure the methods for easier subclassing. I may move the addAdditionalSearchFields() and getAdditionalSearchFieldsUiXml() methods upstream to Store.
Update to use new Store hooks designed to easily add t the search.

Requires https://github.com/silverorange/store/pull/22
Merge pull request #9 from isaacgrant/additional-search-widgets-in-xml

Load search fields from xml instead of building widgets in PHP.