Adds Meteor.disconnect() to allow user disconnection of streams.

As outlined in [this discussion](https://groups.google.com/forum/?hl=en&fromgroups#!searchin/meteor-core/meteor.disconnect/meteor-core/tHc9kC4mjcI/0ktN9qead8EJ) it may occasionally be necessary for the user to disconnect the current stream.

This pull request adds a disconnect method to _DdpClientStream that is modelled on forceDisconnect(). The difference being that it sets the status to 'offline' and allows the user to call reconnect() on the stream later. It also plumbs disconnect into the Meteor. namespace.

Some rudimentary tests are included and the feature has been manually tested to work.