Address issues #175 and #176.  Update pyca-ed25519.

Update the vendor'd pyca-ed25519 to the latest version: https://github.com/pyca/ed25519
Update affected ed25519 modules.

Update modules affected by the changes made to the latest versions of pyca-ed25519 and pyca-pynacl:
Do not use the unsafe key and signature generation functions of pure python ed25519, but do support the signature verification routine.  Developers must use the faster and secure pynacl+libsodium to generate ed25519 keys and signatures.

Temporarily suppress pynacl's import warning error.

Minor edits to comments and code.
Address issues #175 and #176.  Update installation modules.

Add setuptools' find_packages() to avoid repeated updates to the list of packages.
Add setup() arguments, such as classifiers (Trove classifiers used for project classification and searching), keywords, and extra requirements.
Add pip requirements installation file.
Update supported tuf installation options:
Installing from pip and installing an extras requirement (pip install tuf[fast_ed25519]).
Add note to requirements file.
Modify installation setup.

Support the following installation setup:
# Minimal install intended for clients (pure Python, only ed25519 signatures).
pip install tuf

# Optional installation required by the TUF repository tools (RSA keys and rsassa-pss
# signatures, faster ed25519 cryptographic computations, general-purpose cryptography, etc.)
pip install tuf[tools]

Re-implement the fix for issue #167.

Update comments and docstrings related to the fixes above.
Merge remote-tracking branch 'upstream/develop' into develop
Merge branch 'develop', remote-tracking branch 'origin' into develop
Update comments following Trishank's review of the #186 pull request.
Merge pull request #186 from vladimir-v-diaz/develop

Address Issues #175, #176, and the installation of TUF.
Reviewed by Trishank.
[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).
Update README.md
Update README.md
[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.
Fix repository_tool.py issues reported.

Reported Issues:
1. Fix 'files_directory' variable name typo in comment.  Convert get_filepaths_in_directory() to a static method.

2. add_targets() declares 'absolute_list_of_targets' but only appends files to it.  Get rid of this variable.

3. The roleinfo dict of delegated roles was not fully set by load_repository().  Writing new metadata after it had been tried to access a non-existent 'partial_loaded' dict key.  Non-public function that decided whether a metadata file was partially loaded renamed to _metadata_is_partially_loaded(), and now returns a Boolean.

Minor edits to comments related to the fixes above.