Control flow functions from book.mixu.net

For a more detailed discussion of control flow patterns in Node.js see, http://book.mixu.net/ch7.html

Example usage:

var flow = require('controlflow');

# execute tasks as a series
series([
  function(next) { async(1, next); },
  function(next) { async(2, next); },
  function(next) { async(3, next); },
], console.log('Done'); );

# execute all tasks in parallel
parallel([
  function(next) { async(1, next); },
  function(next) { async(2, next); },
  function(next) { async(3, next); },
], console.log('Done'); );

# execute tasks two at a time
limited(2, [
  function(next) { async(1, next); },
  function(next) { async(2, next); },
  function(next) { async(3, next); },
], console.log('Done'); );