Can disable package installation

Fixed #16

I have the same need as describe in the issue but I just don't want dotdeb packages on my server.

With this new attribute it's easy to skip the repository installation.
For now the "repository" recipe is called in the "install" one depending on this value so yes, we need this.
But, if you prefer, I can include the "repository" recipe directly from the default one. It doesn't change the default behavior but those who don't want to install repositories can manually include the install and configure cookbooks.
Both option works but I personally prefer the first one.