Merge pull request #122 from fzs/ldap-deref-alias

Currently the LDAP user service will not dereference aliases when
searching for groups. This patch enables dereferencing aliases for the
group search. This is benefitial if groups are defined in the DIT in a
common place but only certain ones shall play a role in Gitblit. These
can now be linked under a group that can be provided as search base for
groups, without having to recreate the existing groups under the search
base.

In addition, the new doSearch() method implemented in this patch also
limits the attributes returned for the group search to the "cn"
attribute, which is the only one used. That prevents returning all the
members of the result groups, which can be a lot.

Change-Id: I29e1560390810304386dcea5ca40aaf78601b3a9