Run direct and router vegeta runs in parallel.

This achieves 2 things:

* roughly halves how long these tests take to run.
* somewhat eliminates false positives due to differing machine load
  during the 2 runs
[README] Summary of handler behaviours

Brief summary of the features provided by the two handlers that were
recently added.

Remove the S from HTTP since we don't currently support HTTPS.
Merge pull request #51 from alphagov/readme_summarise_handlers

[README] Summary of handler behaviours
Merge pull request #52 from alphagov/parallel_vegeta

Run direct and router vegeta runs in parallel.
Use a non relative latency threshold for perf tests

The problem is that our test backends are ridiculously fast, and
therefore, 200% approximately zero is still approximately zero.

This changes the test to assert that the router doesn't add more than
20ms latency to the requests.
Output latency numbers from tests.

So that we can see what's going on on CI
Merge pull request #53 from alphagov/non_relative_latency_threshold

Use a non relative latency threshold for perf tests
Merge branch 'master' into vendor_mongo_driver