Added "Meteor.call", "Meteor.apply", "Meteor.callInContext"

I would like to add the possibility to call the server methods in unit tests via `Meteor.call` and `Meteor.apply`, what is much less prone to API changes than calling directly in test Meteor.methodMap ....

I have also added two other methods which can set context for the server call `Meteor.callInContext` and `Meteor.applyInContext`