I find using if..else easier to understand than unless with early return

Since we are 'taking in', using the negative logic predicate 'unless yield(elem)' is hard to follow, because that particular elem is the one we are taking in.
Merge pull request #14 from 404pnf/patch-1

I find using if..else easier to understand than unless with early return
Release 0.0.8
+ Add String#ascii_only & #ascii_only!