Add search after bind parameter

This is the first part of the changes I've tried to send yesterday.

I've undone the part which seemed more controversial even to myself,
in the hope these changes could get in with less friction...



This request is meant to replace both

https://github.com/Pylons/pyramid_ldap/pull/3
https://github.com/Pylons/pyramid_ldap/pull/4 

Thank you,

         lorenzo

@anilj: with this pull request,the base dn in itself is templated; I'm not really sure about
adding the means to enable passing an arbitrary dictionary into Connector.authenticate() and
Connector.user_groups(). Some more details on your use-case?