Test for 'Origin by default', plus supporting tests & flags


* Add an '-insecureTLS' option, to disable cert checking
* Add a test to poll the CDN until it has successfully probed Origin 
* Add test code to check that a request will go to origin 'by default' (as in, not in the cache)

How about the following, which looks more like idiomatic loop examples and doesn't keep state outside of loop:
```
for try := 0; try <= maxRetries; try++ {
  …
  resp.StatusCode == 200 {
    break
  }
  if try == maxRetries {
    t.Errorf("CDN still not available after %n attempts", try)
    break
  }
```