Adapt to the new structure

Use class detection to determine what the most sensible defaults are, so
that this extension requires even less configuration.
With recent versions of Symfony, the bootstrap is not required anymore,
nor is requiring the kernel file.
Fixes #140

doc/index.rst

@@ -211,8 +211,9 @@ configure Symfony2 kernel inside Behat to fulfil all your needs.
     or a path relative to the Behat configuration file. Defaults to ``app/autoload.php``.
   - ``path`` - defines the path to the kernel class file in order to instantiate it. It
     can be an absolute path or a path relative to the Behat configuration file. Defaults
-    to ``app/AppKernel.php``.
-  - ``class`` - defines the name of the kernel class. Defaults to ``AppKernel``.
+    to ``app/AppKernel.php`` or ``src/Kernel.php``, whichever exists.
+  - ``class`` - defines the name of the kernel class. Defaults to
+    ``AppKernel`` or ``App\Kernel``, whichever exists.
   - ``env`` - defines the environment in which kernel should be instantiated and used
     inside suite. Defaults to ``test``.
   - ``debug`` - defines whether kernel should be instantiated with ``debug`` option

src/Behat/Symfony2Extension/ServiceContainer/Symfony2Extension.php

@@ -71,9 +71,9 @@ public function configure(ArrayNodeDefinition $builder)
                 ->arrayNode('kernel')
                     ->addDefaultsIfNotSet()
                     ->children()
-                        ->scalarNode('bootstrap')->defaultValue(self::DEFAULT_KERNEL_BOOTSTRAP)->end()
-                        ->scalarNode('path')->defaultValue('app/AppKernel.php')->end()
-                        ->scalarNode('class')->defaultValue('AppKernel')->end()
+                        ->scalarNode('bootstrap')->defaultValue(class_exists('App\Kernel') ? null : self::DEFAULT_KERNEL_BOOTSTRAP)->end()
+                        ->scalarNode('path')->defaultValue(class_exists('App\Kernel') ? null : 'app/AppKernel.php')->end()
+                        ->scalarNode('class')->defaultValue(class_exists('App\Kernel') ? 'App\Kernel' : 'AppKernel')->end()
                         ->scalarNode('env')->defaultValue('test')->end()
                         ->booleanNode('debug')
                             ->beforeNormalization()