Add some basic stack events

[viveklak]

Proposed changes

This change adds support for some basic event publishing. With this change, you can do the following:

$ kubectl describe stacks

...
Events:
  Type     Reason                      Age   From              Message
  ----     ------                      ----  ----              -------
  Warning  StackInitializationFailure  1s    stack-controller  Failed to initialize stack: failed to create and/or select stack: s3backend.s3-op-project: failed to create stack: exit status 255
code: 255
stdout:
stderr: error: could not create stack: An IO error occurred while writing the new snapshot file: blob (key ".pulumi/stacks/s3backend.s3-op-project.json") (code=Unknown): InvalidBucketName: The specified bucket is not valid.
  status code: 400, request id: 9ZT47FVPDT2FR95Y, host id

This should significantly improve the experience of debugging issues with failing stacks.

Related issues (optional)

Fixes #73