too many changes to fit in a single line

- Switch to request because superagent sucks
- Handle more TCP and HTTP errors
- Emit 'complete' with an optional getItems function
- Handle receiving currency (Crowns in Spiral Knights)
- Wait for a response before polling again
- Don't emit 'error' if a user action (like `addItem`) fails
- Remove `cont()`, just let the user restart the trade
- "Public" tradePartnerSteamID
- Ignore trade responses when the trade is over
- Replace `set` method with `sessionID` property and `setCookie` method
- Add `getContexts` and `cancel` methods (closes #1)
- Handle currency in `loadInventory`
- Replace `addItem` with `addItems`, support stackable items
- Don't attempt to re-confirm if the trade is over
- Other things I forgot about