Bug 1096898 - [Usage] Make Usage app work in Mulet

> If you were completely right, building a DEBUG version of Gaia will leave Cost Control unusable.

We'll that is actually true for ``` DEBUG=1 make``` :) Or I'm missing something?

>Which is not the case as if you make GAIA_OPTIMIZE=0 make reset-gaia it continues working. This is because embedding is always performed. Only by commenting line 83 you can make Cost Control fail. This is this way because it is not part of an optimization process but a mock for the <template> tag.

Ok, my fault that I didn't make it clear, ```DEBUG=1 make```  doesn't create app package for device and doesn't use ```webapp-optimize.js``` at all (@julienw please correct me if I'm wrong). It creates profile that actually allows us to use sources directly. ```GAIA_OPTIMIZE=0 make reset-gaia``` as well as ```DEVICE_DEBUG=1 make install-gaia``` do create package for device/b2g and always use ```webapp-optimize.js```. So they're for different puprose.

> Anyway, I think using the LazyLoader is the correct way to do it but I try to remove the sharing code between them but respecting CSS and JavaScript activation. Maybe you should file a bug and try to do this in a separated PR.

Without ```LazyLoader``` CostControl won't work in Nightly\Mulet anyway, don't you mind if I remove duplication (I beleive it's only one ```for``` loop, before js/css activation) in this PR?