Second release

This is quite a major update, with numerous breaking changes.

• separated sync and async Models for clarity (#10)
• reworked batching predict logic: predict no longer automatically switches between single items and batches. Model.predict takes single items, predict_batch only accepts lists of items, and predict_gen knows how to deal with generators (#15, #16, #19, #20)
• caching with native python caches now available (#22, #23)
• documentation improvements