Added ed25519 tests

Added ed25519 tests

Monocypher failed to compile with -DED25519_SHA512 since we added the
incremental interface.  The error has been corrected by the #95 Pull
Request by @vbmithr.

To make sure this doesn't happen again, the test suite has been expanded
to include the official Ed25519 construction.  You can test it thus:

    $ make clean
    $ make test CFLAGS="-DED25519_SHA512 -O3"

Or just run `tests/test.sh` to have the whole shebang.