Changed digitalWrite()'s to direct bit manipulations

I made changes which peekay123 advised to do (https://community.spark.io/t/adafruit-ili9340-library-ported/4010/2) and got some speed improvements.

### Before
```

Adafruit 2.2" SPI TFT Test!
Benchmark                Time (microseconds)
Screen fill              1620498
Text                     284212
Lines                    3092327
Horiz/Vert Lines         140522
Rectangles (outline)     97360
Rectangles (filled)      3367285
Circles (filled)         954738
Circles (outline)        1350160
Triangles (outline)      980730
Triangles (filled)       1341874
Rounded rects (outline)  464334
Rounded rects (filled)   3801234
Done!
```

### After
```
Adafruit 2.2" SPI TFT Test!
Benchmark                Time (microseconds)
Screen fill              1609393
Text                     108868
Lines                    1045099
Horiz/Vert Lines         133730
Rectangles (outline)     86082
Rectangles (filled)      3341967
Circles (filled)         527128
Circles (outline)        456030
Triangles (outline)      331499
Triangles (filled)       1135632
Rounded rects (outline)  196934
Rounded rects (filled)   3654798
Done!
```