DocTestRunner can be modified by end users to override
default testing behaviour when executing doctests. 

Similarly, DocTestCase can be extended to override
default setup/teardown behaviour. 

However, when a DocTestSuite is used to discover
doctests, it is not possible to specify a user-modified
Runner or TestCase.

This patch (against svn revision 46884 of
Lib/doctest.py) enables end users to specify the class
that is used to construct DocTest test cases, and the
runner that is used to run DocTest test cases. 

By default, DocTestSuite will continue to instantiate
DocTestRunner and DocTestCase instances as before.