Memoize ldap entry

This prevents repetitive queries to the LDAP server by memoizing the
LDAP entry in the Connection and model.  It also adds a `ldap_entry`
method for easily getting a reference to the `Net::LDAP::Entry` object.
Fix ldap_get_param when value doesn't exist
Merge pull request #171 from blt04/memoize

Memoize ldap entry