We previously had a baroque inheritance scheme to deal with the case
where the user had normal nose doctests enabled in their environment.
However, this scheme didn't deal with bench() routine, and was
complicated. This commit uses a null Unplugger plugin to pull the
doctest plugin off the nose configuration after it has been initialized.
We can use this for bench() and test(), and it allows the doctest module
to be enabled (by the user environment) and then thrown away.

Also rejigged the docstrings and removed the automated docstring
addition as the docstrings have already been copied and adapted in the
code.

Refactor ``prepare_test_args`` method to make it easier for subclasses
to adapt its behavior.  This should make it easier for nipy and other
projects to use the numpy testing machinery without wholesale copies
into their source trees.

The doctesting tests were in the code file, and (for me) rather
difficult to run without running lots of other tests.  With this change
you can run the doctest tests in isolation by executing the
test_doctesting.py file.

The numpy doctest extension generates an error with empty doctest output;
this most often comes about with the +SKIP option.

The numpy doctest plugin exposed a nose bug because it accidentally used
a different default for the 'doctest-result-variable'.

nose bug report here:
http://code.google.com/p/python-nose/issues/detail?id=445

There are various docstrings show examples of how to run the tests, and
give example test output.  Obviously the test output changes, and
running the doctests for the testing package:

import numpy.testing as npt
npt.test(doctests=True)

will cause several large sets of tests to be run in the rest of the
tree.  So I skipped these.

NumpyDocTestCase definition overwritten further down the file.

The deleted class only redefined the ``id`` method with the same code
as that in the parent class since before nose 0.10.

Move numpy-specific parts of the plugin into their own methods, or into
class-level defines.  This makes it easier to subclass the plugin.  This
in turn may help keep more eyes on the code.