Address Issues #175, #176, and the installation of TUF 

Update the vendor'd pyca-ed25519.  We were using a previous development version.
Update the following modules affected by changes to pynacl and ed25519:
`keys.py`
`ed25519_keys.py`
`test_ed25519_keys.py`
Update `setup.py`, installation options, and address ed25519 import issues.