Add 16bit encode/decode to JPEG2K (WIP)

Add the ability to write 16bit JPEG2k Images (for DICOM medical images).

I'd trying to add a decoder too but I couldn't quite figure it out.  Here's my dumb test case:

```python
import numpy as np
from PIL import Image

arr = np.zeros((64, 96), dtype=np.uint16)
arr[16:64, 0:32] = 300

img = Image.fromarray(arr, 'I;16')
img.save('grr.j2k')  # Works

img2 = Image.open('grr.j2k')
arr2 = np.array(img2)

np.testing.assert_almost_equal(arr, arr2)  # Fails
```

Thoughts?