Add support for virtual attributes.

Use `memberOf` to do operations with groups.
Initialize domains with the ldap object rather than only with the connection.

This will allow us to operate with groups and virtual groups under a specific domain tree.
Allow to override our default virtual attribute names.
Support virtual attributes and subgroups checking users' membership.
Fix readme to reflect the changes in the api.
Merge pull request #16 from github/virtual_attributes

Support for virtual attributes.