Adding in basic httptest support #8

Happy tests
Sad though it brought coverage down 1%. Not entirely sure how. 
```text
$ go test -v
=== RUN TestAuthConfig
--- PASS: TestAuthConfig (0.13 seconds)
=== RUN TestSignRequestNoBody
--- PASS: TestSignRequestNoBody (0.13 seconds)
=== RUN TestSignRequestBody
--- PASS: TestSignRequestBody (0.13 seconds)
=== RUN TestRequest
--- PASS: TestRequest (0.13 seconds)
PASS
ok  	github.com/go-chef/authentication	0.553s

```