NF: robustifying median_otsu for dwi data
Added new tiny volume in dipy/data for tests
NF: now the median_otsu function can have as input non S0 slices and tests with small real dataset were added
Updated with Ariel's suggestions
Replaced with vol_idx as discovered by Matthieu
Tiny commit to wake up Travis
Merge pull request #271 from Garyfallidis/median_otsu_robust

Increased the power of median_otsu masking by allowing to include non S0 volumes to the mask.