NF: add script to build dmg files from buildbots

The buildbots build mpkgs; this script builds dmgs from the mpkgs and
sets the permissions correctly.
NF: add tool for doctest for extensions

Fetched from the dipy/tools directory.
DOC: fix typo; change dipy to nipy

From comments by Bertrand
Merge pull request #267 from matthew-brett/build-dmgs-script

NF: add script to build dmg files from buildbots

The buildbots build mpkgs; this script builds dmgs from the mpkgs and
sets the permissions correctly.