This adds some type safety for Repository.get_file() and

Repository.get_file_exists(), ensuring that the arguments coming in

are all Unicode strings. It then checks that the results from the

hosting service or SCMTool are bytes. This ensures we're always working

with the data types we're expecting.
This required some fixes in a few of the hosting services and SCMTools,

casting string types for comparisons and for the results.

Unit tests pass on Python 2.7 and 3.7 (with other in-progress changes).