v0.2.0

Breaking changes:

• getContexts, loadInventory and getItems will callback with a falsy value in case of a lost session (instead of crashing);
• cancel prevents the 'end' event from being emitted;
• themAssets is now a property, rather than a method (which crashed if _themAssets was empty or sparse).

Other changes:

• The 'end' event now gets 'empty' as the first argument if neither side added any items (only possible when two bots trade).