Add an HTTP test client

This commit adds RspecApiDocumentation::HttpTestClient, which can be used to test *any* website, either on your local development box or out on the internet somewhere.

To use, just add the following line to the top of your spec:

```
let(:client) { RspecApiDocumentation::HttpTestClient.new(self, {host: 'http://yourtestsite.com/'}) }
```