Tentative of lib concatenation in order to avoid multiple http requests
Put lib.js in app-prod since it's for production only
Create a complete build.js
Move PHONY targets together
Use $^ in Makefile
Remove Makefile target .build-artefacts/lib.timestamp

No longer needed now that we concatenate the js files into build.js.
Remove unused APP_LIB_FILES Makefile var
Generate app.js into .build-artefacts

app.js is now an intermediary file, and we no longer need to have it in app-prod
Reorg Makefile rules
Merge pull request #67 from geoadmin/dev_catlib

Lib concatenation