Explicitly name vagrant box as trusty32.

To differentiate from trusty64.
Merge pull request #583 from patcon/patch-1

Explicitly name vagrant box as trusty32.
parse url once
Use new-style classes.
Fix docstrings.
Raise NotImplementedError if BackupTool.restore is invoked.
Avoid os.sep; use os.path.join for buliding paths.

* Also delegate separator handling to os.path.join.
Factor main function.
Catch only OSError.
Cosmetic fixes.
Use classmethods instead of staticmethods.

* Since the name of the class appears inside the method,
  is should be a classmethod, to ease possible future refactoring.
Simplify get_backups.

* Reverse list in-place.
Filter None objects from list.
Refactor get_backup.

* Print warning on bad backup path.
* Strip try..except to absolute minimum.
Ensure tarfile is always closed.
Print error on exception; return after callback.
Do not mess with sensitive magic methods.
Move testing code to appropriate directory.