Release 0.2.0

[njsmith]

• Add nursery.start and nursery.start_soon #298
• server helpers using new nursery.start framework (just tcp and ssl, defer unix domain)
• Deprecations for changes in 0.3.0
  • rework Instrument API to allow efficient implementation #257? – done in Deprecations for #257, plus general deprecation improvements #308
  • Potential API changes to help users who don't realize parenting is a full-time job #136 & How to start up a complex task tree in an orderly way, given spawn's non-determinism? #284 – done in Deprecations for #136 and #284 #309
    • also moving Task, UnboundedQueue into hazmat
  • Use consistent terminology to distinguish functions that run sync callables vs. async callables #68 – see Renames and deprecations for #68 #324
  • Move introspection APIs into hazmat #317 (move introspection APIs into hazmat) – see Move introspection APIs into trio.hazmat #325
  • Drop task management from trio.Queue #321
  • Rename stuff to keep up with new 'checkpoint' nomenclature #157 – see Rename things for consistency with the new "checkpoint" terminology #311
  • Deprecate socket sendall method in favor of SocketStream.send_all #291
  • temporary backcompat for isinstance(x, trio.socket.SocketType)? – done in Deprecations for #136 and #284 #309
• Figure out resolution for Mock network for testing #170 (comment) – see Remove is_trio_socket #326
• decide whether to rename STATUS_IGNORED to TASK_STATUS_IGNORED
• think about whether we want to do anything about Document why SendStream.send_all doesn't automatically use a Lock to serialize calls #328 (but this could be deferred)
• do a pass through the docs, rearranging I/O and tutorial in light of the new networking API; also Some doc todos #240
• Release notes
  • mention that bind will be becoming async (workaround: switch to the high-level API)
• mayyyybe a basic CONTRIBUTING.md (Write a CONTRIBUTING.md #46)?