Add crypto-square exercise

Let's hide all the number manipulation in the `width` function:
```
width(Len) ->
  Sqrt = trunc(math:sqrt(Len)),
  width1( Sqrt, Len).

width1( Sqrt, Len) when Sqrt*Sqrt < Len ->
  Sqrt + 1;
width1( Sqrt, _) ->
  Sqrt.
```
This makes it easier to read the flow of the code.