Fixed #827

Added `Meteor.app_connection` to allow hot code push when connected to
remote server using env: `DDP_DEFAULT_CONNECTION_URL`
Otherwise the app falls into infinit `refresh` loop when setting a remote host.

 `Meteor.app_connection` allways point to the server serving the app bundle - Should it be possible to set this via env too? eg. `DDP_APP_CONNECTION_URL`? *(not sure of its usecase)*

*`Meteor.app_connection` is only set/used clientside*