Adding support for WSSE Authentication callbacks

Added a `soap_config` parameter that specifies a callback used to
authenticate the user and password of a WSSE secured request.

This lambda approach was preferred so as not to introduce too many
changes in the way WSSE already works, while providing the flexibility
to use a default user/password or a custom mechanism for
authentication.
Added module for WSSE parameters in controller

Added `WsseParams` module that enables easy access to `wsse_username` in
controller actions for authentication purposes after the WSSE security
layer.
Merge pull request #137 from sergiocampama/wsse_auth

Adding support for WSSE Authentication callbacks
Little arrangements for #137
Reenabling support for digest based authentication

Reordered the authentication flow so as to check for digest based
authentication before using the specified callback in the controller
Merge pull request #139 from sergiocampama/digest

Reenabling support for digest based authentication
Attempt to fix specs
Fix typo in README.md
Merge pull request #140 from shlensky/patch-1

Fix typo in README.md
Merge branch 'master' of https://github.com/inossidabile/wash_out