Prevent malleable ed25519 and update libsodium

This merges in the contents of PR #30 (updating libsodium, using
its detached signature interface) and adds a check for canonical
signatures, to avoid possible transaction/ledger malleability problems.

The libsodium update was changed to 1.0.0 since that is the newest
presently-available version. I consulted with @jedisct1, and he
recommended we update to it.