add username and password to argument of TOpenSessionReq constructor.

Because Required field 'sessionHandle' is unset! error occurs if pyhs2 connects to hiveserver2 with NOSASL.

No error occurs if it connects to hiveserver2 with NOSASL+username.
Merge pull request #14 from wyukawa/add-username-password

add username and password to argument of TOpenSessionReq constructor.