Use thor in generator

I'm not sure if you like this one...

I replaced the current generator with one based on [Thor](https://github.com/erikhuda/thor).

```bash
$ bin/pliny-generate
```

```
Commands:
  pliny-generate endpoint NAME    # Generates an endpoint
  pliny-generate help [COMMAND]   # Describe available commands or one specific command
  pliny-generate mediator NAME    # Generates a mediator
  pliny-generate migration NAME   # Generates a migration
  pliny-generate model NAME       # Generates a model
  pliny-generate scaffold NAME    # Generates a scaffold of endpoint, model, schema and serializer
  pliny-generate schema NAME      # Generates a schema
  pliny-generate serializer NAME  # Generates a serializer
```

```bash
$ bin/pliny-generate help scaffold
```

```
Usage:
  pliny-generate scaffold NAME

Options:
  [--paranoid], [--no-paranoid]  # adds paranoid support to model

Generates a scaffold of endpoint, model, schema and serializer
```

## ToDo

* [x] pliny: Fix schema rake task