The type of `configuration` object is automatically inferred.

The task delegate override closure properties now match the formatting used in the session delegate. All closure signatures have been updated to match their method counterparts. Finally, all task delegate method implementations are now being overridden by their respective override closure.

Data tasks with a registered progress closure will now always call that closure when data is received as well as update the NSProgress object stored in the `TaskDelegate`. The `Request.progress(_:)` method documentation was also updated to reflect the behavior of data tasks.

This addresses the issues reported in #407.