Add some basic tests (and get them passing)

Author: shoyer

xarray/__init__.py

@@ -1,5 +1,6 @@
 from .core.alignment import align, broadcast, broadcast_arrays
 from .core.combine import concat, auto_combine
+from .core.computation import apply_ufunc
 from .core.extensions import (register_dataarray_accessor,
                               register_dataset_accessor)
 from .core.variable import Variable, Coordinate