This is minor, but I think we should start using the following style when raising exceptions:

```
raise QiimeParseError('message')
```

instead of:

```
raise QiimeParseError, 'message'
```

The former style is recommended in PEP 8 because the latter style is not valid Python 3 syntax. Also, with the former, you don't need ```+\``` since the message is inside parens. Given that we'll be transitioning QIIME to Python 3 at some point in the future, I think it's a good idea to use valid Python 3 syntax wherever possible.

What do you think?