Refactored packages

Refactored package to ease managing them, parsing both XML and JSON ones.

What's new:

* Added complete parsers for both XML and JSON
* Automatic conversion when required
* New command: `info` to get informations about a package (JSON and/or XML)

There are still some things to do:

* [x] Clean code, remove duplication
* [x] Convert package without user interaction by default
* [x] Add a flag to disable automatic conversion
I know that this is not the best way but for now, It seemed OK to me... but OK I'll extract the required code to avoid this.