Adding option to specify/keep quantization tables when saving to JPEG - issue #42

Alex, 

Basically what need to be test is:
 1. is it compiling correctly?
 2. open a JPEG image saved from Photoshop (so it's quantization tables will not be from libjpeg), check the quantization table (`im.quantization`), then save this image with:
```python
im.save('new_name.jpg', quality='keep')
```
Reopen this new image and compare the quantization table to the first one, they should be identical.

Sorry for not having proper tests.