Bring back external responses

[leonard84]

With 106eb93 (https://trello.com/c/UJGIt8j2/28-remove-tika-from-dependencies) the support for external responses was removed. This is quite unfortunate, since we really want this functionality. Tapes become very hard to work with, if the body becomes larger than a certain size (>1000 chars), and in a larger integration test with ~30 interactions and a total size.

Instead of Tika we could just use the Content-Type header to determine the file extension, I even would be ok with a generic .response extension, for the time being.

Removing support for external files due to the Tika dependency and because the yaml files are going to move more towards external

What is meant by 'move more towards external'? The support was just removed.