Helper class for making xhr requests

@fmarier can you take a peek?  This API ok with you?

Adds a wrapper class to make xhr requests easier.

Example:

    var request = new Request(url, creds);
    url.put(data).then( ... );

Supports `put`, `get`, `post`, and `delete`.

I've included a test suite for it, along with fixes to other test suites to make mocha happy.

I rebased over #1 and ended up with a new sha, so my #2 was not automatically updated.  Sorry to re-submit this PR.