More failover tests

Implement:

- TestFailoverErrorPageAllServers5xx
- TestFailoverOriginDownUseFirstMirror
- TestFailoverOriginDownFirstMirrorDownUseSecondMirror

Passing against the mocks and the real CDN:
```
dcarley@ci-slave-4:~/cdn-acceptance-tests$ go test -edgeHost fastly.cdn-acceptance-test.alphagov.co.uk -skipVerifyTLS -v -run 'TestFailoverErrorPageAllServers5xx|TestFailoverOriginDownUseFirstMirror|TestFailoverOriginDownFirstMirrorDownUseSecondMirror'
2014/06/26 08:13:07 Confirming that CDN is healthy
2014/06/26 08:13:07 Started server on port 8082
2014/06/26 08:13:07 Checking health of backup2...
2014/06/26 08:13:15 backup2 is up!
2014/06/26 08:13:15 Started server on port 80812014/06/26 08:13:15 Checking health of backup1...
2014/06/26 08:13:30 backup1 is up!
2014/06/26 08:13:30 Started server on port 8080
2014/06/26 08:13:30 Checking health of origin...
2014/06/26 08:13:41 origin is up!
=== RUN TestFailoverErrorPageAllServers5xx
--- PASS: TestFailoverErrorPageAllServers5xx (0.05 seconds)
=== RUN TestFailoverOriginDownUseFirstMirror
--- PASS: TestFailoverOriginDownUseFirstMirror (0.05 seconds)
=== RUN TestFailoverOriginDownFirstMirrorDownUseSecondMirror
2014/06/26 08:13:41 Started server on port 8080
2014/06/26 08:13:41 Checking health of origin...
2014/06/26 08:13:41 origin is up!
--- PASS: TestFailoverOriginDownFirstMirrorDownUseSecondMirror (0.04 seconds)
PASS
ok      _/home/dcarley/cdn-acceptance-tests     33.471s
```
I'm not sure what the intention of this test is. Is it not the 'Guru meditation' error (depending on the provider) that we're looking for, which is served by the edge when all backends are down?