Refactored structure.

Hi everyone,

Here is my proposal to fix #14, #12 and to improve this gem.

* Move all Symfony related commands into a ```symfony.rake``` file
* Move all deploy hook related stuff into a ```deploy.rake``` file
* Add a new ```symfony:assetic:dump``` command with configurable flags
* Add configurable flags to ```symfony:assets:install``` command
* Improve the doc
* Remove ```symfony:assets:install``` from the default hook structure. For projects using ```SensioDistributionBundle``` this task is already run during composer install.

Thanks for your comments :)