Add the Python implementation of the ed25519 signature scheme

ed25519 is an elliptive-curve public key signature scheme.
http://ed25519.cr.yp.to/
[WIP] Support ed25519 signatures
Add ED25519KEY_SCHEMA to tuf.formats
Continue support for 'ed25519' signatures

Support added for generate(), create_in_metadata_format(), create_from_metadata_format(), create_signature(), and verify_signature().
Add doctest examples
Add ed25519_key.py timing script

Time generate(), create_signature(), and verify_signature().
Perform minor code edits

Add missing letters from the alphabet to 'data' and update import statements.
Add support for PyNaCl

Update generate(), create_signature(), verify_signature() to support PyNaCl.  Caller may optionally generate and create+verify signatures more quickly with 'use_pynacl=True'.
Update time_ed25519.py following addition of PyNaCl support
Add doctests for PyNaCl and fix verify_signature()

If 'use_pynacl=True' is specified and the signature is invalid, verify_signature() raised 'nacl.signing.CryptoError'.  verify_signature() should instead return 'False', to follow the same bahavior of this function when 'use_pynacl=False'.
Modify print message in time_ed25519.py
Update docstrings and comments following pynacl changes
Fix encoding of the seed and signature variable name
Reword comment and fix typo in ed25519_key.py
Merge branch 'develop' of https://github.com/theupdateframework/tuf into ed25519
Update ed25519_key.py following Monzur's review

We now raise an exception instead of blindly passing.  The previous behavior of expecting the caller to verify the availability of PyNaCl is not ideal.

Expanded the comments related to hexlifying data.

Renamed create_in_metadata_format() and create_from_metadata_format().
Merge pull request #131 from theupdateframework/ed25519

Add support for ed25519 signatures.
Reviewed by Monzur.