Add Brewfile
Add .ruby-version
Update README.md to include setup instructions

Extract explanation for the project into `PURPOSE.md`
Cleanup `DirectorySearch`

* Remove `DirectorySearch.server`. Read from `ENV` directly.
* Attempt to cleanup `DirectorySearch#filter`
* Rename `DirectorySearch#search` to `DirectorySearch#results`.
* Remove early returns in `DirectorySearch#results`.
* Remove methods to stub server configuration on `DirectorySearch`. Set
  `ENV['LDAP_SERVER']` in `spec_helper.rb`.