Fix for #1 - incorrect context for partials.

Partials now receive a flattened version of the parent context, including a cloned Mustache instance (if the root context is a Mustache instance). There may be issues with cloning, for example: if a pragma is enabled in the parent template, partials might also inherit the pragma.
Added (basic) test case for partials.

examples/partials/Partials.php

@@ -0,0 +1,13 @@
+<?php
+
+class Partials extends Mustache {
+	public $name = 'ilmich';
+	public $data = array(
+		array('name' => 'federica', 'age' => 27, 'gender' => 'female'),
+		array('name' => 'marco', 'age' => 32, 'gender' => 'male'),
+	);
+
+	protected $_partials = array(
+		'children' => "{{#data}}{{name}} - {{age}} - {{gender}}\n{{/data}}",
+	);
+}