Async Dataset Item ingest

• Dataset.append force argument replaced with update (was inconsistent with documentation)
• Async ingest option available (dataset.append now has an asynchronous flag)
• Async Job object added which can poll status
• Error check for ingesting duplicate reference ids

v0.1.3

• Fixed bug leading to confusing errors on trying to create an empty slice.
• Fixed bug with ndjson serialization.

v0.1.2

• Export autotag scores from dataset object
• Better error surfacing for errors from API
• Bugfixes to GCS urls

April 21st Release

• Improve repr for all objects
• Increase client-side timeout to reduce upload flakiness
• Add slice.items()