BUG: error msg in check_niimgs

The right error was raised, but for the wrong reason
DOC: add example difference estimators
ENH: Add example of multi-class prediction
DOC: multi-class prediction
DOC: iter on estimator choice
No standardization by default in NiftiMasker
DOC: multiple estimators
Standardizing in decoding
TST: in travis, remove Py 2.6

It wasn't working anyhow, because the 'deactivate' was
having us fall back to Py 2.7
ENH: respect dtype in masking
ENH: cache flushing for changing nibabel version
BUG: fix caching bug
ENH: save file in example