Define groups of subject that share common predicates

[afrittoli]

Follow-up to the discussion in #26 (comment)

The spec now distinguishes subjects as:

• long running, which stay in a running state until they're purposely stopped or
  encounter a failure or a condition that prevents them from running - for
  example a service, an environment, an artifact or a source change
• run to completion, which independently stop after they accomplished (or
  failed to) a specific task, or encounter a failure or a condition that
  prevents them from continuing - for example a task run, a build or a test

The terminology does not work for all subjects though, for instance artefacts are not "running".

Some alternatives:

• service vs. job / batch
• permanent vs ephemeral / temporary subject
• add a third category for artefacts

[afrittoli]

Related to #51 - we should try and do these two in the same version of CDEvents.