add ability to attach a signature when creating a commit

src/main/java/org/kohsuke/github/GHCommitBuilder.java

@@ -89,6 +89,19 @@ public class GHCommitBuilder {
         return this;
     }
 
+    /**
+     * Configures the PGP signature of this commit.
+     *
+     * @param signature
+     *            the signature calculated from the commit
+     *
+     * @return the gh commit builder
+     */
+    public GHCommitBuilder withSignature(String signature) {
+        req.with("signature", signature);
+        return this;
+    }
+
     /**
      * Configures the committer of this commit.
      *