message: remove extern crate digest, sha2
diff: -extern crate digest;
  -extern crate sha2;
  -
  -use self::digest::{FixedOutput, Input};
  -use self::sha2::Sha256;
  +use digest::{FixedOutput, Input};
  +use sha2::Sha256;
  use rust_base58::ToBase58;
  use super::{ReadableBlob, Reader, ReaderType};
  