Add support for CSV headers and fix parsing of numbers

[jczaplew]

This addresses one bug and introduces one feature. As with a few other pending pull requests, this introduces the ability to add headers to the CSV output by adding a boolean parameter to res.csv. I have been using this in production for a few months now with no issues.

Additionally, this pull request adds a check to escape to make sure that numbers are properly parsed as such, and are not surrounded by quotes in the output. This has proven useful, as it increases the likelihood that an application like Excel will properly recognize the data type of a field (i.e. numbers are numbers, and not strings).

Thanks!

[xcarpentier]

Maybe it's little bit far-fetched but in waiting (I'm using flatten and lodash too) :

var flat = _.map(contacts, function(item){
 return flatten(item);
});
var keys = _.reduce(_.keys(flat[0]), function(result, item){
 result[item] = item;
 return result;
}, {});
res.csv([keys].concat(flat));

I hope this code help someone... ;-)