Added configuration options

Hi,

First of all, let me thank you for your bundle and library, you have saved me a lot of time lately :)

As an intensive Symfony2 user, I'd like to propose this small PR to enhance the bundle by adding :

* API key parameter validation without the need for a RuntimeException
* default configuration for caching and logging based on common Symfony2 file structure
* new configuration options to enable / disable services (repositories, twig extension) if you don't make use of them
* `symfony/symfony` to composer dependencies

I updated the README, and there should be no BC break, although caching and logging are now enabled by default (but since doctrine cache and monolog are required in composer I don't think it's a problem).

Thanks !
I understand this example intends to state that there's more options then the ones provided though.