Bookmarks export ideas

@jrochkind Here's what I'm thinking for making the bookmarks export work more like the Rails 4 style `MessageEncryptor`. It's very similar to the approach devise took (although they lifted the whole `AS::KeyGenerator` into their codebase). 

Just like devise, I also made the key `Blacklight.secret_key` which defaults to the Rails secret key under Rails 4, but must be set under Rails 3.

Finally, I added the token timeout feature to stop any replayability after 1 hour (we could probably make that 1 minute, but..)