Added convinence method to load grunt tasks from package

Most Gruntfile.js use `grunt.loadNpmTasks(...)` multiple times to load the devDependencies described in the package.json. Also during development, users currently have to do `npm install --save-dev` and then add that task to `GruntFile.js`. 
This task tries to make it easier for most cases. 
This is a convinence method to automatically load all the grunt `devDependencies` defined in `package.json`.