changed code directives to literal blocks

This PR removes reStructuredText "code" directives from the README, changing them Literal Blocks.

".. code" Directives were being used, when the content was not code at all.  Literal Blocks are more appropriate: http://docutils.sourceforge.net/docs/user/rst/quickref.html#literal-blocks

I am hoping an ancillary effect of this PR is that the README will render correctly once published to PyPI again.  You will notice that currently on the page:
https://pypi.python.org/pypi/nose-timer , the reStructuredText markup is displayed raw, rather than parsed and rendered.  I think this is due to an older version of docutils being used, which does not handle code directives.  (GitHub parses it correctly on their repository page, FWIW)