MESSAGE quota resource implemention

[Julien Coloos]

As discussed here and on IRC, I rebased my commits with all the changes:
   - the 'utility' methods have been promoted to 'command' ones, which 
are now generic and may (options hash) concern cyrus binaries
   - the 'start_command_bg' method is now replaced by a 'background' 
option in 'run_command'
   - dropped the 'mode' option, leaving only the 'redirects' one
   - moved 'unpack' method from Cassandane::Unit::TestCase to 
Cassandane::Instance, leaving the tar utility determine itself the kind 
of the tar file
     -> by default destination is the current cassandane instance base 
directory; alternatively one can specify a relative path - 
'path/to/file' - to this directory, or an absolute path  - '/path/to/file'
   - use the pre-existing 'user.cassandane' mailbox for quota upgrade tests
   - fixed a few minor things (like avoiding short-life zombies by 
harvesting some cassandane dead children)
   - refactored a few more stuff
   - removed stuff that became unnecessary

Hoping this will work out this time :)


Regards
Julien