The patch attached provides implementation for multivolume support for tarfile module. It contains both the changes in the module and a battery of unit tests. It contains support for multivolume for both GNU and PAX formats.

The main idea behind this proposal is that for dealing with new volumes, the user will provide a callback function. In this callback function the user typically calls to TarFile.open_volume(filename) with the appropiate filename. This is quite flexible in the sense that the callback function could even ask the user for the filename of the next volume (as tar command does), or do anything they need before returning or before calling to open_volume. 

Please feel free to comment on how to improve the implementation or the API. Documentation for the new feature will be provided at a later stage of the review process if the patch gets a good review.