NF: script to run doctests in extension modules

Nose is not effectively running the doctests in the extension modules.
Basic script to do this sort-of manually.
DOC: use the right function in the doctest strings

Were using ``_squash``, now using ``quick_squash``
BF: fix doctest for val_vec_sum
BF+RF: fix, cleanup distances docstrings

I've changed the docstring test output in some cases.  Can y'all please
check carefully that I haven't certified any wrong results?

I ran the doctest tests with::

    python setup.py build_ext -i && ./tools/doctest_extmods.py dipy