Update (very outdated) dependencies

The only user facing changes are much needed updates to the crypto dependencies, courtesy of #10.

Types from the previous crate, rust-crypto are replaced with types from various crates from https://github.com/RustCrypto.

For example, replace the type crypto::sha2::Sha256 with sha2::Sha256.

Additionally, rustc_serialize has been migrated to serde from #9.