libjpeg-turbo integration

* [x] make libjpeg-turbo compile on Heroku through Vulcan
* [x] update buildpack scripts to download and install libjpeg-turbo during app provisioning
* [x] vendor the script sources, so it's way faster to build
* [ ] perform compile time optimization #5 
* [ ] test everything, a lot

Things that I was not able to do (if anybody can help...):
* Updated libpng to 1.6.7 (still on 1.5.x).

cc @paolochiodi @njvitto

Closes #4