make the first steps towards dualstack by copying tile
build basic dualstack() function, still need to fix offsets
add dualstack shortcut to manpage
Merge branch 'master' into tcl
some fixes for dualstack

* change the method to calculate offsets to loopbased instead of absolute
* fix dualstack in conjunction with borders, still need to do gaps
shorten some code in dualstack
fix dualstack in conjunction with gaps
add dualstack layout to readme