Nanosecond timestamp support

[Mechanical snail]

Duplicity currently has a timestamp granularity of 1 second, whereas current Linux filesystems support nanosecond resolution. Preserving high-resolution timestamps is important for many purposes (e.g. GNU make relies on the full nanosecond timestamps), besides the principle that a backup should preserve everything).

Possibly relevant implementation details: GNU tar does support nanosecond timestamps, but you have to explicitly tell it to create POSIX-format tar files.

Since GNU tar automatically detects the POSIX format when extracting, it should be possible to fix this without breaking backwards compatibility.