Cache requests

[sbfnk]

Implements caching of HTTP requests using the memoise package, closing #51 - caching happens within sessions and documentation has been added for how to preserve surveys between sessions, closing #119

Perhaps an ideal situation would cache between sessions but I don't know how to do that without adding substantial complexity (and writing to the user directory, which they might not want).

[Bisaloo]

I have not used memoise in this way before but after testing locally, it seems to work well indeed, including when the internet is disconnected after cache creation.