Don't try to use bind databag if not specified

Hi !

I used this cookbook without specifiying bind databag, but default recipe still tries to load items from it : 
```
 Net::HTTPServerException
 ------------------------
 404 "Object Not Found"
 
 Cookbook Trace:
 ---------------
   /var/chef/cache/cookbooks/bind/recipes/default.rb:28:in `from_file'
```

Is 'bind' databag really mandatory (it seems not) ?

To fix this I just throw an execption when collecting databag values in default recipe, it's just an idea ... take it or not :-P.

Regards