Adds 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://base.url.of.the.site.you.want.to.test.com/'}) }
```