Add EdDSA (Ed25519) signature support via ed25519-dalek

[flying-kestrel]

This is a PR based on top of #96 that further expands support for EdDSA keys, this time in algorithm::ed25519_dalek, via the ed25519_dalek crate.

The rationale for adding a second mechanism to verify Ed25519 signatures is that OpenSSL is sometimes far too heavyweight of a solution; dalek is also a pure-Rust implementation, which helps for porting to less-supported architectures.

[Dygear]

This is very cool, great job!