Use upstream test client

## What

This PR works against the version of Aspen in the following PR: https://github.com/zetaweb/aspen/pull/149.

Thus it includes changes to bring gittip inline with the latest Aspen changes.

The main purpose of this PR is to use the upstream TestClient found in Aspen.

## Merging

I'd advise caution in merging this into master. I am not involved in the "updating of aspen" inside gittip, but I'd recommend doing something like the following before merging:
  - Merge https://github.com/zetaweb/aspen/pull/149
  - Release a new Aspen (at some point)
  - Upgrade the version of Aspen that gittip is using
  - Some rebasing will need to take place with this PR
  - Merge if travis is green


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