Add a width and height configuration parameter and fix the form theming

so that it works.

DependencyInjection/Configuration.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Gregwar\CaptchaBundle\DependencyInjection;
+
+use Symfony\Component\Config\Definition\Builder\TreeBuilder;
+use Symfony\Component\Config\Definition\ConfigurationInterface;
+
+class Configuration implements ConfigurationInterface
+{
+    /**
+     * Generates the configuration tree.
+     * @return TreeBuilder
+     */
+    public function getConfigTreeBuilder()
+    {
+        $treeBuilder = new TreeBuilder();
+        $rootNode = $treeBuilder->root('gregwar_captcha', 'array');
+        
+        $rootNode
+            ->children()
+                ->arrayNode('image')
+                    ->addDefaultsIfNotSet()
+                    ->children()
+                        ->scalarNode('width')->defaultValue(120)->end()
+                        ->scalarNode('height')->defaultValue(40)->end()
+                    ->end()
+                ->end()
+            ->end()
+        ;
+        return $treeBuilder;
+    }
+}
+