Automate building scratch

[intrigus]

Currently scratch (https://github.com/intrigus/scratch-gui/) (changes are in https://github.com/intrigus/scratch-vm/) has to be manually build and released (e.g. https://github.com/intrigus/scratch-gui/releases/tag/0.2.0.int2).

This can and should be automated.

How to release (should work):

Get the needed libs
mkdir scratch
cd scratch
git clone https://github.com/intrigus/scratch-gui
git clone https://github.com/intrigus/scratch-vm
git clone https://github.com/llk/scratch-blocks # we don't change anything in this repository
cd scratch-vm
npm install
npm link
cd ../scratch-blocks
npm install
npm link
cd ../scratch-gui
npm install
npm link scratch-vm scratch-blocks
npm run-script build
cd build/
find . -name "*.js.map" -type f -delete # These files only add bloat. They are source maps. Might also be possible to not generate them in the first place
tar -cf scratch_dist.tar .
sha256sum scratch_dist.tar > scratch_dist.tar.sha256.txt

upload both scrach_dist.tar and scrach_dist.tar.sha256.txt to the github release page