Add skin aggregation task

[jgribonvald]

Is your feature request related to a problem? Please describe.
The skin aggregation script like implemented in uPortal project is missing here for custom skins if we want to use them in a static way.
i.e. only the portlet aggregator can provide an aggregated skin as the skin.aggr.xml is provided from default skin and isn't builded with customizations.
Due to that we can't include into a skin some custom js (we need to load them from a portlet), they are only included into the skin.xml and not into skin.aggr.xml.
On an other way the system property to disable aggregation don't work anymore (due to systme property not read for this property + property name changed at one place and not on lib dependency about the aggregation

Describe the solution you'd like

• Implements the skin aggregation script
• Reactivate the skin aggregation property use

[jgribonvald]

solved by #253