Merge pull request aws#19 from gkeiser/patch-1

Checking for null...this would have saved me time so I thought I'd submi...

src/main/java/com/amazonaws/auth/BasicAWSCredentials.java

@@ -33,6 +33,12 @@ public class BasicAWSCredentials implements AWSCredentials {
      *            The AWS secret access key.
      */
     public BasicAWSCredentials(String accessKey, String secretKey) {
+        if (accessKey == null) {
+            throw new IllegalArgumentException("Access Key cannot be null.");
+        }
+        if (secretKey == null) {
+            throw new IllegalArgumentException("Secret Key cannot be null.");
+        }
         this.accessKey = accessKey;
         this.secretKey = secretKey;
     }