Search for user DN when BIND_AS_AUTHENTICATING_USER=True

I needed to do this to get user attrs when logging in with an account name. It's a simple fix but I'm not sure it's the best way to go about it. Would like to open up the discussion and possibly implement.

BIND_AS_AUTHENTICATING_USER = True
AUTH_LDAP_USER_DN_TEMPLATE = "%(user)s@your.domain"
AUTH_LDAP_USER_SEARCH = LDAPSearch(
    "ou=USERS,dc=your,dc=domain",
    ldap.SCOPE_SUBTREE,
    "(sAMAccountName=%(user)s)"
)
Match fix for existing open issue "Cant bind and search on ActiveDirectory when using AUTH_LDAP_BIND_AS_AUTHENTICATING_USER"

https://bitbucket.org/psagers/django-auth-ldap/issue/21/cant-bind-and-search-on-activedirectory
Merge pull request #2 from ryangallen/master

Search for user DN when BIND_AS_AUTHENTICATING_USER=True