Code to read the 'users.txt' file from Git, to get 'sponsored' usernames

The getSponsoredUserLogins() method can be pointed at our 'people' repo:

https://github.com/guardian/people

...but for testing, it's just pointed at and clones from a local, minimal
unzipped demo repo, to prove that the parsing code is all working
correctly.
Merge pull request #2 from guardian/read-sponsored-users-from-people-git-repo

Code to read the 'users.txt' file from Git, to get 'sponsored' usernames