RailsInstaller upgrades and new functionalities

List of upgrades / new features:
* Rails 3.2.12
* Ruby 1.9.3-p392
* activerecord-sqlserver-adapter 3.2.12
* DevKit 4.5.2-20111229-1559
* Git 1.8.1.2
* added the possibility to exclude Git from the installation process (maybe it is already installed)