whitespace only fixes to numpy.lowlevel

Fix indentation and trailing whitespace
make OpenCL benchmark run in absence of OpenCL

This may seem a strange thing to do but the numpy-only benchmarks are
still useful. The speedup numbers, however, are meaningless.
remove ipython directive from Sphinx docs

The IPython directive appears broken with IPython 2.0. Since it was only
used in one place, work around it.
Merge pull request #101 from rjw57/remove-ipython

remove ipython directive from Sphinx docs
ensure preserve_backend_stack() preserves backend name

The backend name was incorrectly being modified outside of the with
clause when using preserve_backend_stack().