Add test_repository_tool.py

Add 'test_repository_tool.py' (initial incomplete commit)
Add test cases for the top-level metadata objects (Root(), Targets(), etc.)
Add test case for Repository()
Add test case for get_target_hash()
Minor update to 'repository_tool.py': add default rolename for Targets() and call tuf.util.get_target_hash() in the
repository tool's get_target_hash()