Change CharSets from map[int][]string to [][]string
Changing character set no longer can cause panic

Before, if a new charset was given such that the new one had less
characters than the original one, there would be a panic when
s.CharSet[count] was accessed. By removing spinner.Offset, and just
using the length of the array in place (same speed), the need to update
the Offset field is removed at no time cost, and len(CharSet) fills the
same role
Fix doc comment for CharSets
Merge pull request #3 from millere/master

Fix panics when changing charset