Merge pull request #185 from vladimir-v-diaz/develop

Remove find-code.sh
Note: We are replacing find-code.sh because git-grep does the same, and more. Also, a development script unrelated to TUF should probably not be included in the source. Developers who may modify the source are likely to have their preferred way of doing things.
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.