Change the default dataset download directory to ~/nilearn_data

* Tweak the tests to use absolute paths and put the temporary data in
  tmpdir so that it gets cleaned up during teardwon. Also remove PEP8
  warnings.
* Update doc mentionning nilearn_data
Merge pull request #281 from lesteve/default-nilearn-data-in-home

Change the default dataset download directory to ~/nilearn_data