Fix Shell::nl() to be outputting newlines instead of returning.

Either the code or the documentation should be fixed. 

`Shell::nl()` was earlier returning number of linefeeds while the documentation (http://book.cakephp.org/2.0/en/console-and-shells.html#Shell::nl) says it should be outputting.

For consistency, along with other methods in `Shell`, especially `hr()`, I believe outputting instead of returning would be more appropriate.