Added overridable methods for returing authz and token endpoints.
Added self-acknowledgement as requested in btilly/perl-oauth2#1.
Fixed copying required/optional params in collect_action_params.
Allow additional arguments to be passed to the save_tokens callback.

This change allows an arbitrary list of arguments to be specified during LWP::Authen::OAuth2 object construction, and those arguments are then passed to the save_tokens callback in addition to the token string.
rearranged Changelog, added new changes
added contributors, bumped version