- turn initial cred optional in bind_sasl. It is missing for some methods.
  this fixes a wireshark warning that ldap packet was corrupted
- added support for SASL authentication with gem ruby-sasl. This allows
  DIGEST-MD5, GSS-SPNEGO, GSSAPI authentication.
- unified @conn.bind call to conn_bind method
- appended :host information for bind auth args as it is necessary
  for some SASL mechs
- added support for SASL security layer (similar to SSL). Currently
  only for gssapi/kerberos, but changing only ruby-sasl, it might be easy
  to implement.