Add function to move cursor between screens

Here are three functions that enables moving cursor between screen.

Example usage:
``` lua
local cursor    = require "mjolnir.jstevenson.cursor"

function throwCursor()
  local nextScreen = cursor.screen():next()
  local nextFrame = nextScreen:fullframe()
  local x = nextFrame.w / 2
  local y = nextFrame.h / 2
  cursor.movetopoint(nextScreen, x, y)
  cursor.show(nextScreen)
end

hotkey.bind({"cmd"}, "k", throwCursor)
```