Fix IPython.start_kernel()

Closes gh-4005
Fix passing user_ns to IPython.start_ipython()
Fix passing user_ns to IPython.start_kernel()
Use traitlet on InteractiveShellApp instead of storing user_ns via __init__
Add REPL-like printing to %%R cell magic
Fix warning when running IPython.kernel tests

The signature of TemporaryDirectory.cleanup() changed just before Python 3.2, and our
TemporaryWorkingDirectory subclass was receiving an unexpected parameter. This copies
in a newer TemporaryDirectory.cleanup() implementation for Python 2.7.
Use onlyif_unicode_paths in utils tests
Refine unicode tests for ultratb
Skip some more tests that require unicode paths
made docstrings clearer and more explicit/correct for eval and R methods
Use print_function in IPython.utils.tempdir
moved printing logic to the R magic function (out of the eval function)
removed unnecessary parentheses
Fix parallel.client.View map() on numpy arrays
Fix test_map_iterable()
DOC: fix links to matplotlib, notebook docs

I've added a link to pandas, made all matplotlib links point to
matplotlib.org, and fixed a link in the notebook help menu to point to
our new notebook.html, now that htmlnotebook.html is a redirect.

Whoever ends up merging this PR should backport it to 1.x.
small doc fix in nbconvert
Fix test_map_iterable() again
Add test for IPython.start_kernel()
prepend :meth: Docstrings are not markdown.