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

Hope this is useful @lindseydew - 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 all works correctly.

The code is a bit hacked together, but hopefully an nice small example of reading data from Git?