make cluster work with cordova

Hi @arunoda, I found a bug in `meteorhacks:cluster` two days ago, `DDP.connect()` always use `'http://meteor.local'` as prefix to construct url in cordova, so the `Cluster.discoverConnection()` can not work. Then I tried to fix that and committed a pull request yesterday. Maybe you can take a look and merge that to the master branch?
