[lib] Split Bridge/Cache/Format into one file per class

The files have grown to a size where it is necessary to search
for a class in a file. This commit splits the content into one
file per class. RSS-Bridge will require implementations and
the implementations will require (once) the interfaces.

lib/FormatInterface.php

@@ -0,0 +1,6 @@
+<?php
+interface FormatInterface{
+    public function stringify();
+    public function display();
+    public function setItems(array $bridges);
+}