Access site with invalid SSL

This is a pull request to implement the functionality described in https://github.com/cantino/huginn/issues/648. This is still lacking docs. As there're no docs on the WebRequestConcern itself (AFAIK), we'll need to document the `disable_ssl_verification` on all agent that implements this concern.

Is it right, @cantino? Is there a better way? Apart from the docs, any thoughts on this PR?