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 cask data from git trees.

There are a few consequences which do not strictly pertain cask counting and searching.
##### On features 
We can no longer link to the HTML view (on GitHub) of cask definition files from the search results; at least, not without additional requests.

However, for each cask, we now receive an url to the base64-encoded blob. If we were so inclined, we could use it to render the cask's content directly.

##### On 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.