sortable functionality (without tests!)

Guys, I found when running Dredd that the order of the methods was causing problems. 

Specifically, in our [API docs](http://docs.activecellserver.apiary.io/) we describe collections after models, and therefore `POST` commands occur _after_ `GET` commands. This is problematic because we're asking for objects before they are created.

This PR provides a `--sorted` option that sorts the transactions by method, preventing this problem. In our case, the change is enough to go from failing tests to passing tests.

I am not familiar with the tools that you use for testing, so I realize that this PR doesn't include tests. I did, however, document the method. If you find this useful, please feel free to merge!

Great library. We really love it.