Got git targets working (when they're pointing to github).

As a stop-gap to getting git working completely in tavern, I've used
Github's APIs to download a tar.gz of the master/head.

For example, the following pubspec works:

name: git-test
dependencies:
  test_tavern_library:
    git:
      url: git@github.com/Pat-Laub/test_tavern_library

Note: only github repos will work, and only those with a valid pubspec
file.

R=tapted@chromium.org

Review URL: https://codereview.appspot.com/58100043