Move common commands into own module.
Moved class Repository into own module.
Fix error in get_expired_backups().

Before, the search for expired backups was cancelled for a specific
interval if not backups were expired due to the "keep" option, so
"keep_age" was ignored.
Remove debug print statement.
Remove debug print statement.
Move interval parsing function into own module.
Moved constants into own module.
Fix wrong import.
Change importing of constants.

Instead of importing all constants with wildcards, every constant will
be prefixed with the module name as usual.
Fix calculation of expired backups by age.

Now the maximum ages will be recomputed in every poll cycle instead of
once at the start of the program, which leads to wrongly marking expired
backups as not expired if they expired after startup.
Add license headers to source files.
Huge refactoring.

- Add docstring to nearly all modules, classes and functions.
- Rename cmd.py to files.py.
- Lots of little stuff.
Merge branch 'refactor' into develop