Modified callback interface for decoded form data - BREAKS COMPATIBILITY!
Modified callback interface for decoded form data - BREAKS COMPATIBILITY!

Given the lack of documentation for this feature it's unlikely it's
being used and there's no reason to carry a broken interface into the
future or otherwise provide a migration strategy for users.

If this commit breaks your data handler callbacks, simply modify your
callbacks as follows:

Before:

```
handle_data(Data, Env) ->
    ...
```

After:

```
handle_data({ok, Data}, Env) ->
    ...
```

If you want to handle content that's not decodable because it's
missing or has an unexpected content type, provide this clause:

```
handle_data({error, {content_type, Type, Body}}, Env) ->
    ...
```

gar1t/psycho