Abstract the call to `dsq_urlopen()` into its own method

The `DisqusAPI::call()` method calls `dsq_urlopen()` directly. We'd like to abstract this into its own method so we can extend the `DisqusAPI` class and override it in order to use our own HTTP API rather than `dsq_urlopen()`.

Specifically, we'd like this implemented so the Disqus plugin for WordPress can switch to using the WordPress HTTP API for HTTP calls.