Refactor app.js to make it smaller and split into parts based on tasks.

[adelura]

When we change something in samples, and want to see final result we have to run builder. As we all know it takes some time. We could split build task into logical parts (I think it's already done) and allow us to run only one task using CLI. For example building only samples might be an option. Also we could make builder Gruntable and add some watcher to let things in background.

[oleq]

dev/builder/app.js should be split into separate sub-tasks, based on grunt-contrib-* (and others) rather than promises and additional libraries (i.e. https://github.com/gruntjs/grunt-contrib-copy instead of ncp).

Atomic tasks should be placed in short, separate files under tasks/ folder (http://www.thomasboyt.com/2013/09/01/maintainable-grunt.html) because of the complexity of app.js.