BF+TST: bug in nifti convert for 'unknown' space

'unknown' space should work if the header matches, but if there is no
header, the zooms weren't being set, and the check was failing.  Fix and
test.
TST: add coveralls automated coverage

Set up coveralls automated coverage report.  Based partly on
https://github.com/scipy/scipy/pulls/2907
TST: enable slow tests for travis
Merge pull request #297 from matthew-brett/add-coveralls

TST: add coveralls automated coverage

Set up coveralls automated coverage report. Based partly on 
https://github.com/scipy/scipy/pulls/2907
Merge pull request #294 from matthew-brett/fix-nifti-ref-no-header

BF+TST: bug in nifti convert for 'unknown' space

'unknown' space should work if the header matches, but if there is no header,
the zooms weren't being set, and the check was failing. Fix and test.