Filter parameters config with expressions

This pull is related to https://github.com/doctrine/mongodb-odm/issues/899
It allows set filter parameters in config file.
Parse value parameter by [ExpressionLanguage](http://symfony.com/doc/current/components/expression_language/syntax.html)
Looking at [Creating a New ExpressionLanguage Class](http://symfony.com/doc/current/components/expression_language/extending.html#creating-a-new-expressionlanguage-class), I'm not sure how the examples in https://github.com/doctrine/mongodb-odm/issues/899#issuecomment-46082507 will be implemented here. Since we're constructing a new ExpressionLanguage, it looks like Symfony's `parameter()` function and any custom functions would not be available to the user.

The only other example I found in Symfony for an ExpressionLanguage with custom functions was within SecurityBundle (using an Authorization ExpressionLanguage instance). There was also an ExpressionLanguage in the DependencyInjection component, but I don't see that used anywhere.

If `constant()` is the only special function available to users, I'm not if there's much benefit to implementing this.