tests: Expand Stored Procedure invocation tests.

- Show it's also possible to call SPs with output parameters by using
  the helper pymssql.putput class.
- Show it's possible to fetch results of queries performed by SPs
  both from _mssql and pymssql.
pymssql: Made handling of integer SP params more robust.
Made test compatible with Python 3.
Merge pull request #237 from pymssql/more-sp-tests

tests: Expand Stored Procedure invocation tests.