[R-pkgs] new version of trackObjs

[Tony Plate]

The trackObjs package stores objects in files on disk so that files are
automatically rewritten when objects are changed, and so
that objects are accessible but do not occupy memory until
they are accessed. Also tracks times when objects are created
and modified, and caches some basic characteristics of objects
to allow for fast summaries of objects.

This version trackObjs_0.8-0 fixes some bugs:

     o   Fixed faulty detection of conflicting existing objects
         when starting to track to an existing directory.

     o   Replaced environment on function that is in the active
         binding for a tracked object.  Previously, that function
         could, if constructed via track(obj <- value), have a
         copy of the tracked object in its environment, which would
         stay present taking up memory even if the object was
         flushed out of the tracking environment.

     o   Fixed bug that stopped track.stop(all=TRUE) from working

-- Tony Plate