Register plugins on "system.ready" event

* This allows (PHPUnit) tests to be written for plugins
because their paths will be in the include path. The previous
method of registering them prevents this as it relies on request
routing and this does not always take place when PHPUnit tests
are run
Uppercase the request_method once and for all
Delete one or more settings parameters
Change constructor and DSSG_Api::instance signatures

* Optional api_url parameter in constructor

* Accessor and mutator methods for the http_client
and api_url properties
Tests for the DSSG_Api class

* TODO: Extend tests to check for status code
and the response body (from the API call)