tweak, add auth, update readme

Hi,

I used this package for one of my recent projects, and it worked out exceedingly well for my needs. I especially like your use of caching and how you kept the code minimal. I tweaked it a bit to enhance its functionality:

1. Updated Readme to describe installation and common use-cases
2. Added 'auth' method to perform basic ldap authentication
3. Added config parameters 'key' and 'userdn' so users can modify these based on their ldap implementation
4. get() retrieves all attribs if no parameter is given
5. Minor tweaks and comments
6. Added composer dependency on ext_ldap
7. Created tag 1.1

I hope you find these edits useful.

Thanks!