JPG and PNG decoder

[bearophile]

cal:

> I've been working on decoders for simple (baseline) JPEG and 
> PNG's, mostly for my own amusement, and they seem to work ok 
> now, so if anyone has need of some simple D modules to load 
> these formats you can grab them here:
> https://github.com/callumenator/imaged

Suggestions on the code:
- Try to use final switches.
- switch cases don't need (), so instead of case(foo):  write 
case foo:
- Try to add const/immutable/pure/nothrow where possible.

Bye,
bearophile