Underscore private variables

[Swaagie]

To stay in line with the server side part of bigpipe, we should underscore private variabeles. This should also allow 'collision free' property usage for client side plugins.

Currently the following would fail in client side pagelets due to not having _bigpipe on the client side. This would go for all general modules or constructors that are used isomorphic.

function DataStore(pagelet) {
  if (!this) return new DataStore(pagelet);

  this.readable('_pagelet', pagelet);
  this.readable('_pouchdb', pagelet._bigpipe.pouchdb);