Implemented attribute filtering

[schtibe]

Implemented functionality that filters the returned attributes on a search like the python-ldap module does.

[timabbott]

Hi @schtibe! The Zulip project just adopted this package which had been idle for a long time, so if you have time to clean up and rebase this PR, we can consider it for inclusion.

[zulipbot]

Heads up @schtibe, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.

[sunweaver]

@schtibe: Please rebase this branch against current master and explain in the commit message in more detail what the proposed changes do.

I'd be happy to see inline comments for the proposed changes.

[schtibe]

I'm sorry, I lost track of this PR a long time ago. Closing it.