Refactored the controller to read variables from the request

This ensures that the variable values used to dump the asset are the one
requested for this url instead of expecting that the value supplier is in
the same state than when generating the url for the asset.
This assumption is already an issue for the default supplier: the
symfony environment is likely to be the same, but the request locale is
based on matching _locale, not locale. So the controller will generally
use the en locale unless a listener changed it.
Merge pull request #250 from stof/controller_asset_variables

Refactored the controller to read variables from the request