Implements case where a map in ReadMap contains arrays.

I was trying to read `map[string]interface{}` in fatherhood and had the bad surprise of a `panic: Not supported yet` =). I saw the `ReadMap` method on scanner was not able to read arrays, so I fixed that.

I've added a `ReadSlice` method on the `Scanner` interface, for consistency.

Would this make megajson support arrays of any types?

r: @benbjohnson