Remove Assetic in favor of Gulp

[egeloen]

This PR removes Assetic in favor of Gulp. As you can see, this approach allows us to remove lot of codes and make the assets management much more easier.

[blazarecki]

You can have one file per task it's more readable thanks to wrench

var gulp = require('gulp');
var wrench = require('wrench');

/**
 *  This will load all js or coffee files in the gulp directory
 *  in order to load all gulp tasks
 */
wrench.readdirSyncRecursive('./gulp').filter(function(file) {
  return (/\.(js|coffee)$/i).test(file);
}).map(function(file) {
  require('./gulp/' + file);
});

[egeloen]

Thanks a lot for the feedback. I was not aware of this gulp module but as the current script is pretty light and the tasks are too, I would prefer keep everything in a single file and if tomorrow, it becomes too fat, I will split them using wrench.