MSSQLConnection: Add `with` (context manager) support

Add context manager support to `MSSQLConnection` for using `with` statement ([PEP 343](http://www.python.org/dev/peps/pep-0343/)).

Continues context manager work done for `pymssql.Connection` and `pymssql.Cursor` in https://github.com/pymssql/pymssql/commit/b9ef4ba2

This will be handy to have in tests to get rid of explicit `close` calls.

e.g.:

```python
        conn = mssqlconn()
        conn.execute_query("SELECT @@version AS version")
        conn.close()
```

can become:

```python
        with mssqlconn() as conn:
            conn.execute_query("SELECT @@version AS version")
```

Cc: @rsyring, @damoxc, @ramiro