ENH: Adding brains module

ENH: Added segmentation volume support

ENH: Added posterior volume support
ENH: Added methods for accumulated posterior files

BUG: Corrected default threshold for binary volume calculations
BUG: Grad dirs were not correctly rotated w/ trans
Merge pull request #174 from dmwelch/module

ENH: Adding brains module
Merge pull request #184 from BRAINSia/FixResampleDWIInPlace

BUG: Grad dirs were not correctly rotated w/ trans