Added "pie.moveSlice" to offset a slice by given values

- manipulates Slice._offsetX + Slice._offsetY
- if an offset is given to Method "Pie.createSlice()", the most outer
div of the created Slice will have its z-index increased by 1 to resolve
issue of overlapping after moving
Introducing Attribute of Pie "_startingPercentage" - "firstPercentage()"

- holds the value at wich Slices can start
- used for "Pie.update()" to set percentages of slices correctly
- used in library methods as starting Percentage Value
- default = 0
- no setter Method yet!
removing alerts