Remove hardcode commands

This now dynamically populates the commands available for each data type in qiita_pet.

It also adds functionality to the Command object to pull out a dictionary of commands keyed to the datatype they are for.
why not just change line 368 to:

```python
results = [(convert_to_id(dt, "data_type", conn_handler), None)
               for dt in datatypes]
```

That way you don't have to unpack and you don't have to `zip`