[WIP] Assetic/uglifyjs cookbook article

This PR involves a new cookbook describing how to use uglifyjs with assetics.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.0+
| Fixed tickets | #1841,#1099

Right now, I copied the yuicomposer cookbook and changed it tto uglifyjs where needed. Please suggest further changes to improve this cookbook. 

Pleae also have a look at how the installation of ugliffyjs is done and if this fits "our" view of how it should be installed.