Package defaults

My implementation of the discussed feature at #116: Package defaults and logging of the package options (defaults merged with additional options).

Not sure if I'd implemented in the way it should, mostly because I got a bit confused in the Package module (`package.rb`), but it works.

<del>There should probably also be a test that verifies the merging of options and defaults</del>

Example log:
```
[production]
  * git
    --> already installed for roles: app
  * deploy_user
    --> already installed for roles: app
  * rbenv
    └─ ruby_version: 1.9.3-p392
    --> already installed for roles: app
```