Set version to 0.2dev.
Add Jinja2 requirement to readme.
Rename mux/demux to scatter/gather.

The terms were used for the opposite meaning (multiplex() should have
been named demultiplex(), and vice versa). To avoid further confusion,
the intuitive names gather() and scatter() are now used.
Delete obsolete comment.
Conform to PEP 8 formatting.

(Sorry, lots of spurious changes, but will likely be better in the long
run.)
Comment update (space.Cache).
Add unit testing with py.test.

runtests.py created by py.test 2.3.4 (edited slightly to conform to PEP
8).

ndmake/__init__.py added, since otherwise tests fail.
Rename a method (invalidate_computations).

Also remove a redundant statement.
Improve representation of mtime extrema.

Stop using magic number 0, which was technically (though not
practically) a bug.
Tidy up mtime handling.

Introduce constant and function for missing-files condition.
Rename private methods in space.Cache.
Eliminate possible race from files.touch().

Not that it is likely to cause any problems.
Eliminate vertex ids from depgraph.
Slightly optimize cycle checking.
Fix upside-down GraphViz output.