Added/modified features:
- Commands 'add' and 'update' simplified to just one: 'add'. Comand 'update' is no longer available. This fulfills most of issue #25.
- New command 'rebuild-index', which rebuilds either the general or the specific indices for packages, based on the current packages actually downloaded. Useful in case some index has been corrupted or is not coherent with what is currently in the local machine (issue #35).