feat(JitsiConference): add 'getConnection' getter

Adds a getter to retrieve JitsiConnection instance used by
the JitsiConference.

JitsiConference.js

@@ -441,6 +441,13 @@ JitsiConference.prototype.getName = function() {
     return this.options.name;
 };
 
+/**
+ * Returns the {@link JitsiConnection} used by this this conference.
+ */
+JitsiConference.prototype.getConnection = function() {
+    return this.connection;
+};
+
 /**
  * Check if authentication is enabled for this conference.
  */