- Fix problems with test decorators when used on test generators.  
- The skip/fail arguments for skipif and knownfailureif can now be 
  either a bool or a callable that returns a bool.
- Added tests for the test decorators. 

which allows flagging tests as known failures rather than skips.
Updated test_umath to use knownfailureif.

of explicit imports or dependency on the local scope where the doctest is 
defined..

Remove unnecessary "import re".
Limit doctest execution environment to "import numpy as np".
Save and restore print options after each doctest (to clean up after some doctests that change 
them).
Enable ellipsis for all doctests.
Remove parameter to NumpyDocTestCase constructor that was specific to nose 0.11.
Monkeypatch wantFile of doctest plugin to skip Python files related to the build process 
(scons_support.py and generate_numpy_api.py).

(prevent import of nose until actual execution of tests).  Restored 
"raises" function to numpy/testing/utils.py until it can be replaced with the function of 
the same name from nose.tools after the lazy import.