NF: add setuptools-aware package checking

Add extra dictionary arg to package_check so that package_check knows
its working with setuptools, and can fill the dependencies in the
dictionary accordingly.
RF: adapt setup.py dependency checks to setuptools.

This is still a disaster if running normal ``python setup_egg.py
install`` - but no more of a disaster than before.
Merge pull request #234 from nipy/package_check-fix

MRG: adapt package_check to setuptools

Now pip is working pretty well, adapt package_check to fill setuptools
arguments instead of giving errors, when setuptools has been imported.