Split up DSL into resource/endpoint/callback concerns. Ref #28

[samwgoldman]

No attempt was made to clean up the individual DSLs, but this should be a decent starting point to better define the boundaries.

One implication of this change is that the following is now illegal syntax:

resource "Orders" do
  example_request "Getting an order" do
    # assertions
  end
end

This is a good thing, because the user has not specified what the endpoint should be.

[oestrich]

Now to find a way to split up the dsl_spec file.