Deletes shares from server 0 correctly

Deletes the shares from server 0 without setting the remaining
size to 0
Adds delete_all_shares
Fixes tests which were improperly deleting shares
Removes unnecessary import
Merge branch markberger/ticket-2008

This changes test_upload.py to delete-all-shares more gently, leaving
the 'incoming' directory in place, which should help the rest of the
test pass.

Closes tahoe-lafs/tahoe-lafs#50 (github PR)
Closes ticket:2008 (trac)