Update to MiddleMan 4.x compatibility

[xunker]

Make branch at current master for 3.x compatibility, master moves to 4.x.
Create release tags for new releases.
Consider moving version number to match middleman version number, with our version number embedded. Something like: 4.x.y.a.b.c, where 4, x, and y are middleman version numbers and a, b, and c are our version numbers. Maybe we only need a and b, no c?