v0.6.1

This release transitions to using a more formalized schema called Chronicle Schema

Schema-related changes

• Remove internal models (activity.rb, etc) and use models from Chronicle Core
• add global cli flags --schema and --format to control schema and serialization
• connector plugins transform to Chronicle Schema by default
• move responsibility of serialization to Chronicle Core

Other changes

• Use rubocop rules from chronicle-core
• Support multiple sequential transformations in a job
• Add buffer, fields_limit, filter_fields, filter, format (for serialization), sampler, sort, transformers