TST fix string labels in metrics tests

Also found a faulty format op in the error handling.
COSMIT
Update what's new
DOC state that string is possible
TST with labels arguments
FIX what's new...
use new virtualenv features of travis, so we don't have to kill the virtualenv
DOC copyedit preprocessing docs
Merge pull request #2228 from amueller/travis_virtualenv_stuff

use new virtualenv features of travis,
Trying to enable python 3.3 too.
Update .travis.yml
MAINT ignore profiling results from kernprof.py
DOC copyedit KernelCenterer docstring
DOC minimal kernel centering narrative docs
fix: newest numpy doesn't like all-indexing non-existing dimension (reported by erg #2233)
ENH remove bad examples
DOC let some example for prf metrics
Merge remote-tracking branch 'upstream/master'
FIX: Newer numpy causes scipy to issue a DeprecationWarning. Ignore it. Fixes #2234.
ENH allows make_multilabel_classification to return label indicator f…