added the concept of a main DIC extension for bundles
added the concept of a main DIC extension for bundles

This allows for better conventions and better error messages if you
use the wrong configuration alias in a config file.

This is also the first step for a bigger refactoring of how the configuration
works (see next commits).

 * Bundle::registerExtensions() method has been renamed to Bundle::build()

 * The "main" DIC extension must be renamed to the new convention to be
   automatically registered:

      SensioBlogBundle -> DependencyInjection\SensioBlogExtension

 * The main DIC extension alias must follow the convention:

      sensio_blog for SensioBlogBundle

 * If you have more than one extension for a bundle (which should really
   never be the case), they must be registered manually by overriding the
   build() method

 * If you use YAML or PHP for your configuration, renamed the following
   configuration entry points in your configs:

      app -> framework
      webprofiler -> web_profiler
      doctrine_odm -> doctrine_mongo_db

edpauto/Yaml,teohhanhui/Yaml,symfony/web-profiler-bundle,symfony/web-profiler-bundle,nVitius/Yaml,superdav42/Yaml