Update frame.dart

Fix: The imported libraries 'sockjsadapter.dart' and 'frame.dart' should not have the same name '' 
This error makes impossible to use Frame and StompAdapter classes together, then you import it like
import 'package:stompdart/sockjsadapter.dart' as StompAdapter;
import 'package:stompdart/frame.dart' as Frame;
Merge pull request #6 from Dumbris/patch-1

Merged pull request.