Integrated Distill library

The installer now integrates the [Distill library](https://github.com/raulfraile/distill) to download the best format (ZIP or TGZ) and to uncompress it using the native operating system commands:

![symfony-installer](https://cloud.githubusercontent.com/assets/73419/5088468/330ba72c-6f34-11e4-9fdc-89e462dd296b.gif)
I think it's more clear to use `getcwd()` instead of `$this->projectDir`. Otherwise, the message will be something like "There is already a 'foo' project in this directory (/var/www/projects/foo)."