[WIP] Refactor test_updater.py and repository_setup.py.

Add generate.py script and pre-generated repository files used by Unit tests (replacing repository_setup.py).
Refactor test_updater.py (10 test cases remaining).
[WIP] Refactor test_updater.py

Continue refactoring the test cases of test_updater.py.
Fix issue where repository_tool.py was not writing new compressed metadata.
Minor edits to TUF modules.
Merge branch 'develop' of https://github.com/vladimir-v-diaz/tuf into develop
Finish refactor of test_updater.py.

test_updater.py now uses repository_tool.py to update the remote repository as needed, and no longer depends on mocking and repository_setup.py.
Remove obsolete helper functions.

Minor edits to comments.
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().
Move repository data to 'tuf/tests/repository_data'
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 develop
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
Re-add comment about disabling password confirmation in repository_tool.py.
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.