Change the ipython printing test to use init_printing().

This removes the following deprecation warning while running the tests:

sympy/interactive/tests/test_ipythonprinting.py[1] sympy/utilities/exceptions.py:149:
SymPyDeprecationWarning:

using %load_ext sympy.interactive.ipythonprinting has been deprecated
since SymPy 0.7.3. Use from sympy import init_printing ;
init_printing() instead. See
http://code.google.com/p/sympy/issues/detail?id=3914 for more info.

  warning(see_above, SymPyDeprecationWarning)
.                                                            [OK]
Skip plotting doctest in sympy/geometry/ellipse.py

Fixes issue 3925: Failing doctest in sympy/geometry/ellipse
http://code.google.com/p/sympy/issues/detail?id=3925
Fix cancel() of a Piecewise().

Fixes issue 3923: Calling cancel() on a Piecewise() instance.
http://code.google.com/p/sympy/issues/detail?id=3923
Merge pull request #2244 from jrioux/piecewise

Fix cancel() of a Piecewise().
Merge pull request #2246 from jrioux/interactive

Skip plotting doctest and avoid deprecation warning.