WebHandler redesign

[lhazlewood]

We've currently imposed a new interface on a user WebHandler (which is maybe misnamed because it is Account-specific?). Maybe we can use a generic Servlet Filter approach for this to reduce new concepts. Need to evaluate first before releasing 1.0 final.

spec: https://github.com/stormpath/stormpath-framework-spec/blob/master/handlers.md

(although we shouldn't call these Handlers because they're not handling anything - they're pre/post logic where the concept of a Filter is a better fit).