Trn 126

The content of the ticket is (https://jira2.iparadigms.com/browse/TRN-126)

Although we don't currently intend to store our response data inside walker, it would make walker far more useful to a broad audience and should greatly help project adoption. It should also be very easy to implement.
Basically:
Add a config flag datastore.store_response_body, false by default
Add a column body to walker.links
If the flag is true, have cassandra.Datastore store the response body in that text column
Since this could be important to others, update the README to note that Walker can do this
Good idea. I'll update the comments.

One thing that has come up with docs before is as follows. When I'm writing docs, I can refer to the config variable in question as cassandra.store_response_body. Not sure if we want to be more generic and use $datastore.store_response_body (?). I'm assuming not, but we may need to consider that if more datastores start getting used.