ENH: remove warnings with default settings

IMHO default settings should never raise warnings
[ENH] allow specification of vmin in plot_stat_map

This allows plotting e.g. p-values with a sensible use of the colorbar. Proposed after discussion with @mrahim and @lesteve
Revert "[ENH] allow specification of vmin in plot_stat_map"

This reverts commit 165591a64dad335581a9881844d23fd673c84a41.
Merge branch 'master' of github.com:dohmatob/nilearn