Modified version of patch #5281 by Joshua Leung.
It's a two part patch:
Add a Roll button in the transform properties floating panel with bones selected in edit mode
Add a Roll transform (Ctrl-R) that can be used to modify the roll of selected bones in edit mode
The transformation modifies the roll of all selected bones incrementally (like a rotation does to the bones' rotation) [that is the part that differs from the patch. The patch would set the same roll value to all bones]
Also, this commit includes some shuffling around of the functions, to keep the previously logical order. :)
f72a56bdfc