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
This API is weird. Your parser class is both the parser and the result of the parsing. It is not only a parser (and most of its API is actually being a package, not a parser)