This is a big commit - It should have been done on several

This is a big commit - It should have been done on several
branches:

1) I've started work on a FileSystem implementation, which
invovled fgactoring out some common code.
2) This was partly superceded by adding blob support
directly to the persistentence manager
3) I've added some tools to import/export the repository into
a zip file. This is a work in progress: its not possible
to unzip the zip file.