Retrieve casks with Git Data API

A solution to #27 is to switch to the [Git Database API](https://developer.github.com/v3/git/) and obtain the cask data from git trees.

#### Features
We can no longer link to the HTML view (on GitHub) of the related cask from the search results; at least, not without additional requests.

However, the git tree object includes an url to the base64-encoded blob. If we were so inclined, we could use it to render the contents of cask definition files.

#### Page Loading
Some approximated network changes for circa 1750 casks:
11 requests, 1MB → 12 requests, 800KB

The net effect should be a marginal improvement in loading times.