Use relative imports (or absolute imports for cimport)

Use relative imports (or absolute imports for cimport)

The relative-import ticket is still open [1], but the Cython 0.15
release notes (2011-08-05) claim support for relative imports [2].
Relative cimports are not supported yet [3], but I've fixed them up to
use the full absolute import path.

[1]: http://trac.cython.org/cython_trac/ticket/542
[2]: http://wiki.cython.org/ReleaseNotes-0.15
[3]: http://thread.gmane.org/gmane.comp.python.cython.devel/14406