TileJSON load should call callback(err, data)

Fixes #189.

Use sinon-server for Fake XHR. Adding the assets directly; they're
not available via npm.

We can't actually test JSONP; it doesn't actually use XHR. We'd
need a server component that responds to JSONP requests with the
correct content. A stub-based approach would likely produce a test
that in practice could never fail.