gigasecond: add stub with type signature (#306)

The signature of the function is not at all obvious.

exercises/gigasecond/src/lib.rs

@@ -0,0 +1,7 @@
+extern crate chrono;
+use chrono::*;
+
+// Returns a UTC DateTime one billion seconds after start.
+pub fn after(start: DateTime<UTC>) -> DateTime<UTC> {
+    unimplemented!()
+}