Add a build check for PRs (#51)

This now checks that distribution files are unmodified and fails otherwise. Also checks if the author is present in the NOTICE file and prints the result, but as email addresses may vary, does not hard-fail.

src/types.ts

@@ -541,7 +541,7 @@ export class Signature {
     // check return type
     var thisReturnType = this.returnType;
     var targetReturnType = target.returnType;
-    return thisReturnType == targetReturnType || this.returnType.isAssignableTo(target.returnType);
+    return thisReturnType == targetReturnType || thisReturnType.isAssignableTo(targetReturnType);
   }
 
   /** Converts this signature to a function type string. */