Added Migration Generator

@jdoconnor 

I figured out how to do subcommands in Thor, so now we can do:

```
Commands:
  napa generate api <api_name>              # Create a Grape API, Model and Entity
  napa generate migration <migration_name>  # Create a Database Migration
  napa help [COMMAND]                       # Describe available commands or one specific command
  napa new <app_name> [app_path]            # Create a scaffold for a new Napa service
  napa version                              # Shows the Napa version number
```