Patch from Jaggz H, thanks!
[#31096] Weight-painting: Brush-specific weights
http://projects.blender.org/tracker/?func=detail&atid=127&aid=31096&group_id=9
Each brush's weight can now be set individually, can also enable
unified setting (same as size and strength have.)
Added readfile code to the patch: subversion bumped to 1, brushes get
default weight of 0.5, unified weight enabled by default and value
from old vgroup_weight field.