Added rudimentary store searching and model for products in the store

This patch adds the ability to use the unofficial store API at `https://www.humblebundle.com/store/api/humblebundle` to search the store for products, or even get a list all items in the store.

It is not yet feature complete as it was purely to fill my need to get and compare the prices of products in the store. It would be possible to extend it to search based upon DRM and OS in future, and potentially even purchase products.