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