RF: replace numpy.testing.assert_ with assert_true

numpy.testing.assert_ appeared in numpy 1.3.  That's pretty old, but
it's the only thing stopping us being compatible with numpy 1.2, and
nose assert_true is just as readable.
RF: no dict methods for old numpy load returns

Older numpies (at least up until 1.3) return a dict-like thing from
``np.load`` on an ``npz`` file, but without an ``items`` method or other
helpful iteration stuff.  Fall back to the ``files`` attribute on the
returned object to find the first and only array to get the design
matrix.
RF: avoid import error for numpy 1.2

measure function not present in numpy 1.2; avoid import error during
tests by deferring name lookup until benchmarks are run.  They will
still error in numpy 1.2 when benchmarking, but having the benchmarks
run is more of a luxury than having the tests run without error.
Merge pull request #252 from matthew-brett/old-numpy-compat

Old numpy compat

Make nipy compatible with numpy 1.2