Binary values support

Hi,

First I'd like to thank you for your awesome work !

While using PyWldap in a project, I needed to fetch binary values from a LDAP server. Here is the patch that I used to add support for those.

I tried to follow the code-style you used and actually had to manual merge my changes after the recent code reorganization. Please let me know if I need to change anything for the PR to be accepted.

Cheers