Allow customisation of the distribution plugin

Hi,

I've implemented the following stories  ( from https://github.com/gradle/gradle/blob/master/design-docs/building-and-packaging-applications-and-libraries.md ) : 
Allow customisation of the distribution plugin
Test coverage
Allow distributions to be installed
Generate a TAR distribution.

I'd like to have few feedbacks before going on the next stories.

Cheers,
Sébastien