Added concat and uglify tasks to grunt. 

Piskel on master will only be able to work in debug mode, by passing ?debug in URL. The minified version is built in /build, which has been added to .gitignore.

In this version piskel can now be attacked from 3 distinct URLs : 
- http://domain:port/piskel/ -> packaged + minified version, only available after running grunt
- http://domain:port/piskel/?pack  -> packaged version, only available after running grunt
- http://domain:port/piskel/?debug -> same as before, always available

@grosbouddha I need some feedback on this. I know we can always wait until we start doing come packaging / minification. My main goal here is to simplify the synchronisation between the version I have on app-engine, and the one we have here. (and minification doesn't hurt)

I know there are other solutions for my app-engine issue here. I could have a build on the app-engine side that clones the latest piskel version on github, builds it and deploys it. That would even be cleaner I guess (since app-engine is the "aggregator" here).

So if you want we can keep the github project simple, no packaging, no minification. Tell me how you feel about this.