Add Result alias (#271)

Adds a `yubikey::Result` alias with `yubikey::Error` as the error type.

Since we only have one `Error` type, this simplifies the return types
where a `Result` is returned.

src/lib.rs

@@ -149,7 +149,7 @@ mod transaction;
 pub mod yubikey;
 
 pub use self::{
-    error::Error,
+    error::{Error, Result},
     key::Key,
     mgm::MgmKey,
     readers::Readers,