added ABC Problem

Added two implementations of the ABC problem.
@utkarshkukreti I think it's because `contains` is defined on `T:PartialEq` but `Chars` (the struct returned by `chars()`) does not implement `PartialEq`. The `contains` defined on str slices instead wants a `&str` argument.
I found two options: `collect()`-ing chars in a `Vec` before using `contains()` or transforming the char to a string slice. None of them looks particularly nice to me... what do you think?

http://is.gd/Xzd8gl