Pass the request as a parameter to the preprocessor

This would allow us to access other arguments of the request when implementing preprocessors.

In my specific case, I'm trying to preprocess a request sent by a Backgrid.js based frontend which sets the parameters of the request as strings using a one-by-one mapping from fontend state parameters.  By accessing these string parameters I could then build in the preprocessor the `search_params` dict that flask-restless is expecting.

In a more general case this gives more power to the preprocessors.