Add docs on how to use via public functions

[bradoyler]

Thought it might be a common use-case to use this module via its "API".

Like so:

var myConfig = require('./config.json');
myConfig.mongodb.password = process.env.MONGODB_PASSWORD;
myConfig.s3.secret = process.env.S3_SECRET;
var mongoBackup = require('mongodb_s3_backup');

// inside cron\task
mongoBackup.sync(myConfig.mongodb, myConfig.s3);

@theycallmeswift This seems to work, but not sure if I'm missing anything.