Add :secure option to force HTTPS

`ThreeScale::Client.new(provider_key: '...', secure: true, persistent: true)`

Please note that it might be slower than http version.

Fixes #12 

It introduces `ThreeScale::HTTPClient` because `net-http-persistent` has different API from net/http.
In the future it could wrap other http client libraries.

The `persistent: true` requires `net-http-persistent` gem.