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

[johnbillion]

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.

[simonwheatley]

We've made these choices, abstracting dsq_open() to a method so we can override it, in the hope that they will work for other PHP systems which use the Disqus PHP libraries and have their own HTTP APIs, e.g. Drupal, Joomla, etc.