NF: add script_test attribute to script tests

We need to be able to skip the script tests to make the bdist-egg-tests
Makefile target work.
BF: update Makefile targets for testing

The testing commands were breaking because:

* doctests were failing from ``nibabel.test()`` because of numpy's
  defult behavior of stripping the namespace before running the doctest.
* The bdist-egg-tests target fails because we now have script tests, and
  these need the new bin directory on the path.  This is too annoying to
  implement, so just script the script tests.
RF: use PYTHON variable for python binary

Use variable to allow user to set the python binary used for tests etc.
NF: add makefile target for installing from sdist
Merge pull request #199 from matthew-brett/makefile-updates

MRG: updates to Makefile targets and testing

Automated testing from the Makefile was broken; fix and add new testing
Makefile target.