Core installer

I've added the magento-core package type with installation and update routines, allowing you to set up the whole project with composer.

Using either https://github.com/bragento/magento-core or http://packages.firegento.com (as soon as my pull request for the magento/core repo gets through) you can now require "magento/core" which has the "magento-core" compoyser package-type.

The added pre/post Install/Update routines for this package type allow for a fully automated Magento Installation and seamless Update processes.

1. Installation:

- Require magento/core
- If no magento-root-dir defined you are prompted to define one
- If your magento-root-dir is missing, you are prompted if you want to have one created automatically
- You can have your composer root file require any magento-module packages besides the core
- During the installation the root-dir is emptied in order to bypass possible filesystem hickups with symlinks
- After the core installation 
-- the magento-modules which may have been installed up to that point are redeployed with the configured deploy strategy
-- the folder rights required by the Magento core are set (by default [app/etc, media, var])

2. Update:

- While updating certain files and folders need to be persistent to guarantee a seamless upgrade without data loss (by default [app/etc/local.xml, media, var])
- During the update process a tmp root dir is created where a fresh core is installed first
- Afterwards the modules get redeployed there again
- Finally the folder rights are set again
- The persistent folders are moved into the updated core 
- The "var/cache" folder is cleared
- You are prompted and get a final chance to check if any files in the original root dir need to be transferred to the updated root
- The backup dir gets cleaned up


My first tests were very promising. Now I would like to involve the community to round things up.