MAINT remove redundant class hierarchy + fix api perform parameter check in fit
DOC fix example in univariate selection (fix issue #3132)
FIX testing datasets have too few feature for default of SelectKBest
TST improve parameter checking + use nose assert_ functions
DOC remove comment thanks to @jnothman

'All our feature selectors output masks now (as they should, IMO).
It will be turned back into indices only for sparse matrix indexing,
but only until we require scipy >= 0.14 (?).'
Merge branch 'simplify-fselection'

Fixes #3131.