Revert "Merge pull request #7001 from shoyer/NaT-comparison"

This reverts commit 7141f40b58ed1e7071cde78ab7bc8ab37e9c5983,
reversing changes made to 8fa6e3bef26a6d4a2c92f2824129aa4409be2590.

The original broke some pandas tests. The current plan to get this
in is

* reversion
* issue FutureWarning in 1.11 and 1.12
* make the change in 1.13.

Now, NaT compares like NaN:
- NaT != NaT -> True
- NaT == NaT (and all other comparisons) -> False

We discussed this on the mailing list back in October:
https://mail.scipy.org/pipermail/numpy-discussion/2015-October/073968.html

The test is in numpy/lib/tests/test_io.py. This commit is intended
as a demonstration of using temppath.

Context manager intended for use when the same temporary file needs to
be opened and closed more than once. The context manager creates the
file, closes it, and returns the path to the file. On exit from the
context block the file is removed. The file should be closed before
exiting the context as an error will be raised on windows if not.

Also fix up the `tempdir` context manager to deal with exceptions.

Tests are added for both `temppath` and `tempdir`.

* use SkipTest in numpy tests instead of importing it from nose
* add a KnownFailureException as an alias for KnownFailureTest
  (the former is preferred, but the latter is kept for backcompat)
* rename the KnownFailure nose plugin into KnownFailurePlugin,
  and keep the old name for backcompat