Issue 3666 context out of xhr

@6a68 - yo dude, I am trying to pull the context stuff out of xhr and move it into network.js. It has gone well, but I could use some collaboration time.

Specifically, https://github.com/shane-tomlinson/browserid/compare/issue_3666_context_out_of_xhr?expand=1#L1R53.  I want to change this to:

`get({`

but doing so has repercussions that I did not foresee. The old code requested session_context and returned results would be returned immediately instead of being deferred. This is the *only* request that acted like this. I want to make this request consistent with all other requests and have its completion deferred. 

Some tests depend on this behavior, the tests are in `user.js` and `state.js`. You can see in the first commit the changes I had to make to the tests. I was a bit uncomfortable doing this without some input to see if the scope of the PR is expanding too far.

This is a continuation of mozilla/browserid#3655 to address mozilla/browserid#3666.