Transpose z-index like a floors

[Jabher]

Map in accordance with http://en.wikipedia.org/wiki/Storey#Numbering

.a {z-index: ground;}
.b {z-index: 1;}

to

.a {z-index: 1;}
.b {z-index: 2;}

[hashanp]

Great idea! I don't want to confuse anyone too much though, so what I think I'll do is I'll have another property called storey. E.g.:

.a {storey:ground;}
.b {storey:1;}

is mapped to:

.a {z-index:1;}
.b {z-index:2;}

[igoradamenko]

@HashanP You can close it, 'cause it was implemented in #10 :3