introduce a basic distribution plugin.

Hi,

I implemented the basic distribution plugin 
 
https://github.com/gradle/gradle/blob/master/design-docs/building-and-packaging-applications-and-libraries.md#introduce-a-basic-distribution-plugin

I introduce the distribution plugin and change the java-library-plugin to use the distribution plugin.

I'd like to have few feedbacks before beginning development of the customisation of distribution plugin.

Cheers,
Sébastien