Remove replace get-pixels module by jpeg-js

[pmdartus]

#8 made the lib JPEG centric.

In order to make the lib lighter and reduce the external dependencies, it could be interesting to switch the get-pixels module by the jpeg-js module. https://github.com/scijs/get-pixels/blob/master/package.json#L13

[paulirish]

Nice! Slimming that down would be super awesome.