Cargo API

Here is a new API called cargo, it is like queue with single worker

**queue with concurrency 2**
![queue](https://f.cloud.github.com/assets/1676871/68108/bbc0cfb0-5f29-11e2-974f-3397c64dc858.gif)


**cargo with payload 2**
![cargo](https://f.cloud.github.com/assets/1676871/68101/84c92066-5f29-11e2-814f-9d3d02413bfd.gif)