Add Misc Header and Auth support [WIP]

This adds support for authorization and misc headers.
Misc Headers can be added in the remote connector settings
in client/datasource.json.
Authorization happens automatically using local/session storage.
While headers are tested and working, authorization requires
two other PRs to work.

  1. Loopback needs to use User.remoteMethod instead of
     loopback.remoteMethod in common/models/user.js
     see https://github.com/strongloop/loopback/pull/943

  2. Strong-Remoting needs to support hooks when
     using http.prototype.invoke
     see https://github.com/strongloop/strong-remoting/pull/152

See https://github.com/strongloop/strong-remoting/issues/105