Modify get_row() to use the CPython tuple API

This drops the previous method of building up a row tuple and switches
to using the CPython API, which allows you to create a correctly sized
tuple at the beginning and simply fill it in. This appears to offer
around a 10% boost when fetching rows from a table where the data is
already in memory.

The CPU usage is still up around 100% so more optimisations should be
able to be made somewhere if some proper profiling was done.

#### Without patch
```
$ python2 test_pymssql.py
Grabbed 936222 row(s) in 10.957s
Grabbed 936222 row(s) in 10.728s
Grabbed 936222 row(s) in 10.669s
Grabbed 936222 row(s) in 10.610s
Grabbed 936222 row(s) in 10.738s
Grabbed 936222 row(s) in 10.620s
Grabbed 936222 row(s) in 10.855s
Grabbed 936222 row(s) in 10.844s
Grabbed 936222 row(s) in 10.603s
Grabbed 936222 row(s) in 10.697s
Max: 10.957s
Min: 10.603s
Avg: 10.732s
```

#### With patch
```
$ python2 test_pymssql.py
Grabbed 936222 row(s) in 9.676s
Grabbed 936222 row(s) in 9.537s
Grabbed 936222 row(s) in 9.481s
Grabbed 936222 row(s) in 9.424s
Grabbed 936222 row(s) in 9.281s
Grabbed 936222 row(s) in 9.421s
Grabbed 936222 row(s) in 9.291s
Grabbed 936222 row(s) in 9.454s
Grabbed 936222 row(s) in 9.438s
Grabbed 936222 row(s) in 9.443s
Max: 9.676s
Min: 9.281s
Avg: 9.445s
```