Add missing test cases to test_util.py.

Add missing test cases for the following functions:
get_target_hash()
find_delegated_role()
paths_are_consistent_with_hash_prefixes()
ensure_all_targets_allowed()

Add test condition to test_B6_load_json_string().
Refactor test_arbitrary_package_attack.

Refactored to use the 'unittest' module (test conditions in code, rather
than verifying text output), use pre-generated repository files, and
discontinue use of the old repository tools.  Fix for issue #111.
Merge branch 'develop' of https://github.com/theupdateframework/tuf into add_test_cases_to_test_util
Merge branch 'add_test_cases_to_test_util' of https://github.com/vladimir-v-diaz/tuf into add_test_cases_to_test_util
Edit comments and minor code cleanup.

Edit test_arbitrary_package_attack.py
Merge branch 'develop' of https://github.com/theupdateframework/tuf into refactor_test_arbitrary_package_attacks
Initial refactor of test_indefinite_freeze_attack.py
Minor edits to comments.
Merge pull request #189 from vladimir-v-diaz/develop

Refactor test_updater.py
Merge pull request #192 from vladimir-v-diaz/refactor_test_arbitrary_package_attacks

Refactor test_arbitrary_package_attack.
Remove old 'tuf/tests/unit/test_repository' directory.
Resolve Merge.
Merge pull request #191 from vladimir-v-diaz/add_test_cases_to_test_util

Add missing test cases to test_util.py.
Merge branch 'develop' of https://github.com/theupdateframework/tuf into refactor_test_indefinite_freeze_attack
Re-add repository data for the tests.
Merge pull request #194 from vladimir-v-diaz/refactor_test_indefinite_freeze_attack

Refactor test indefinite freeze attack
Refactor test_endless_data_attack.py

Refactored 'test_endless_data_attack.py' to use the 'unittest' module (test
conditions in code, rather than verifying text output), use pre-generated
repository files, and discontinue use of the old repository tools. Minor
edits to the test cases.
Merge pull request #195 from vladimir-v-diaz/refactor_test_endless_data_attack

Refactor test_endless_data_attack.py
Update test_indefinite_freeze_attack.py

Minor edit to comment.
Disable console log messages enabled by the repository tool, which this unit tests needs.
Sleep for at least 1 minute before checking for an expired metadata error, and ensure the function supplied to self.assertraises() is a callable.