Minor clean-up of the FWHM to sigma conversion

No need to use 'fwhm' on both sides of the = sign and the comment about
not using /= with numpy scalar wasn't entirely helping.
Merge pull request #283 from lesteve/fwhm-to-sigma-conversion-tweak

[MRG] Minor clean-up of the FWHM to sigma conversion
Merge branch 'master' of https://github.com/dohmatob/nilearn
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
Merge branch 'master' into sparse-models
REFACTOR: in space_net.py cv scores are 'bigger is better'