#493 definition['type'] throws exception

 definition['type'] == 'media', since this seems to be a check if media fields are present, I think the get() method should be better. I had some problems with a 
nested key
'items': {'type': 'list', 'schema': {'type': 'dict', 'schema': '{another dict}'}
Merge branch 'develop' of github.com:alanderex/eve into alanderex-develop
Occasional KeyError building _media helper. #271.
 alanderex
Merge branch 'alanderex-develop' into develop