tests: Initial connect test and docs

Adds an extremely basic initial test to ensure that we are able to
connect to a YubiKey.

The test is marked `#[ignore]` in the hope that we can eventually start
adding tests which run in CI, e.g. against a mock card.

This also includes a fix for calculating the APDU size, since the ones
we were sending originally were overly long.

tests/integration.rs

@@ -0,0 +1,20 @@
+//! Integration tests
+
+#![forbid(unsafe_code)]
+#![warn(missing_docs, rust_2018_idioms, trivial_casts, unused_qualifications)]
+
+use std::env;
+use yubikey_piv::YubiKey;
+
+#[test]
+#[ignore]
+fn connect() {
+    // Only show logs if `RUST_LOG` is set
+    if env::var("RUST_LOG").is_ok() {
+        env_logger::builder().format_timestamp(None).init();
+    }
+
+    let mut yubikey = YubiKey::open(None).unwrap();
+    dbg!(&yubikey.version());
+    dbg!(&yubikey.serial());
+}