Bug pandas compat asserts

fixes test asserts that are failing because of changes in pandas master, and numpy.testing.assert_equal doesn't work anymore.

e.g. https://launchpadlibrarian.net/182311320/buildlog_ubuntu-utopic-i386.statsmodels_0.6.0~ppa18~revno-1690~ubuntu14.10.1_UPLOADING.txt.gz

this adds a `statsmodels.tools.testing` module with an assert_equal wrapper for the corresponding numpy and pandas functions.

changes 1 test in `test_arima` so that desired is an Index not a list of strings.

`statsmodels.tools.testing` is currently used only for one case, namely to have backwards compatibility with `assert_index_equal` and `assert_equal` for older pandas