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

Refactor test_arbitrary_package_attack.