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.