Install profile inheritance

Issue Summary

This was brought up on this reply (http://drupal.org/node/509404#comment-1838990)

To make install profiles easier to write, and more powerful tools in general, we should allow install profiles to inherit from each other, similarly to how sub themes work.

Currently when developing an install profile, you start by copying the default profile and editing what you don't want and then branch off permanently into your own little playground.

What would be very useful is to be able to specify a 'parent' install profile in the .info file for the profile and then extend the existing functionality.

A good example of this would be Open Atrium or Acquia.

Developers could create a new install profile, that builds on and further specializes an existing platform (ie: open atrium for open source developers etc, open atrium for scrums).

Developers could even build a base profile for all their clients, and extend it with client specific functionality.

This code would build on pretty easily from #509398: Install profiles should be modules with full access to the Drupal API and all it entails(.install files, dependencies, update_x)

It would also improve #525594: Installation should consist of 2 phases instead of one
By allowing you to develop an install profile that simply provides a simple client facing install wizard.