LB-1394 Load all grunt tasks in one line with load-grunt-tasks
LB-1394 Create separate jshint tasks to lint or not the libs

Libs often have a lot of jshint errors, it's useful to be able to
separate them from backbone files.
Merge pull request #8 from patriciagarcia/LB-1394

LB-1394 Improvements to Gruntfile.js