Fix fmt verb typo in helper error message

Found by `go vet`:

    ➜  cdn-acceptance-tests git:(master) ✗ go vet
    helpers.go:88: unrecognized printf verb 'n'
    exit status 1

Should be a base10 integer. Otherwise the following message is generated:

    2014/06/12 11:55:50 CDN still not available after %!n(int=20) attempts
Add LICENSE

Because: open source. Generated with:

    lice -o 'Crown (Government Digital Service)' mit > LICENSE
Merge pull request #31 from alphagov/add_license

Add LICENSE
Some README guidelines for writing tests

Based on what we've learnt so far.
Document how to selectively run tests

This assumes consistent naming described in the preceding commit. Still
doesn't answer how we make people aware of all our flags, per #21.
Merge pull request #30 from alphagov/fmt_verb_typo

Fix fmt verb typo in helper error message
[#19] Add some more unimplemented NoCache tests

These assert the standard behaviour of any CDN. Mostly cribbed from the
default Varnish configuration:

https://github.com/varnish/Varnish-Cache/blob/3.0/bin/varnishd/default.vcl
Implement TestNoCachePOST

This looks very similar to TestNoCacheHeaderSetCookie. I suspect most of the
other tests in this category will.
Add testThreeRequestsNotCached helper

Since all of the NoCache tests are going to follow the same pattern of
sending three requests and verify that we get three unique responses. The
only things that change will be:

- request: to modify the method or change request headers. So this is
  constructed outside and passed in by each test.
- response headers: to set headers like `Set-Cookie`. So a callback can be
  passed which modifies the headers (any method you would normally call on
  http.Header) before the response is written.

This should make all of the subsequent tests much more succinct.
Implement TestNoCacheHeaderAuthorization

Set an `Authorization` header on the response.
Implement TestNoCacheHeaderCookie

Set a `Cookie` header on the request.
Implement TestNoCacheHeaderCacheControlPrivate

Set a `Cache-Control: private` header on the response.
Mock for Cache-Control: private

This isn't default Varnish behaviour per:

https://www.varnish-cache.org/trac/ticket/477

However Fastly have implemented it in their standard config and I'm led to
believe that CloudFlare do the same, so we'll keep it in and mock it for
now. If it turns out that CloudFlare don't do it then we should consider
moving the tests away from the "standard caching behaviour" category.
Merge pull request #29 from alphagov/implement_standard_nocache_tests

Implement all NoCache tests
Merge pull request #32 from alphagov/readme_writing_tests

Documentation for writing and running tests
Log varnish syntax errors on reload

So that you can see what went wrong with the new configuration without
needing to log into the Vagrant VM.
Implement test(s) for X-Forwarded-For

I've decided to collapse the two tests together because it's much easier to
construct the expected value of the "append" test when we know what our
reported IP address is from the "create" test. Compared to testing the
number of items and whether they are each a valid IP address.

This test currently fails against our Fastly configuration because we appear
to be appending the same IP twice. I'll raise a bug in Pivotal for that:

    cdn@gds-cdn-noodle:~/dan$ go test -edgeHost fastly.cdn-acceptance-test.alphagov.co.uk -skipVerifyTLS -v -run XFF
    2014/06/13 05:43:40 Confirming that CDN is healthy
    === RUN TestHeaderXFFCreateAndAppend
    --- FAIL: TestHeaderXFFCreateAndAppend (0.00 seconds)
            cdn_acceptance_test.go:86: Expected origin to receive "X-Forwarded-For" header with single IP. Got "80.240.128.97, 80.240.128.97"
Merge pull request #34 from alphagov/varnish_reload_log

Log varnish syntax errors on reload
Merge pull request #35 from alphagov/xff_header

Implement test(s) for X-Forwarded-For
Merge branch 'master' into age_header