Basic API client with swipe uploading

Needs more planning and work WRT how to make failing authentication work.

Uploads swipes one at a time. We need to tag them with some sort of UUID so that the Bark API servers can avoid doubling up of same swipes.

Client needs to truncate the local `swipes` file after sending -- not implemented for safety's sake.

Client does not work out of the box -- I send in `event_description` instead of `event_id`. Need to change the DB schema in `csesoc/bark-core` and the `/swipe` POST handler to support that. The idea is for `event_id`s to be resolved later in the web interface.