Rename combineAttributeWithRects() > combineAttributeAndSignals()
Replace -offsetX:Y: with -offsetByAmount:towardEdge:

This is in keeping with using NSLayoutAttribute for
coordinate-system-independent directions.
Added convenience offsetting methods
Disallow extending center X or center Y

-offsetByAmount:towardEdge: should be used instead, to not be dependent
on the coordinate system.
Clarify the behavior of extending width or height
Merge branch 'master' into better-offset
Merge pull request #47 from github/better-offset

Replace -offsetX:Y: with NSLayoutAttribute support, disallow center X/Y extension