Feature request: Allow for remote Push server

Hi all,
I am really liking the work you did in this library. However it does not allow to implement my scenario. I want to be able to create a separate meteor server for only sending push notifications.

This is the scenario I would like to achieve.

However, in [this](https://github.com/raix/push/blob/master/lib/client/client.js) file, there are several calls done as `Meteor.call`, which will always use the server assiciated to the client.

This forces me to implement it as this:
The stuff marked in red is what is wrong with the current setup.

Would it be possible to allow the client to configure a `Connection` which can be used for registering on instead of always using the `Meteor.call`.

Start with lib/client/client.js, especially the Meteor.call usages described in the issue, and trace how client registration currently reaches the server. Review the library's existing client connection setup and determine the configuration boundary for a separate Connection. Done means registration can target a separately configured push server without relying on the client-associated Meteor server, with coverage for the changed calls.

Written by the indexing model from the issue text.