Merge pull request #32 from negbie/patch-1

ParseBytes returns *cgraph.Graph and error

README.md

@@ -83,7 +83,7 @@ b, err := ioutil.ReadFile(path)
 if err != nil {
   log.Fatal(err)
 }
-graph := graphviz.ParseBytes(b)
+graph, err := graphviz.ParseBytes(b)
 ```
 
 ## 3. Render Graph