Destinations.addGlobalFields allows adding global fields to all messages.
Public API for adding global fields to output.
Fix duplicate test method names.
Documentation of eliot.add_global_fields.
Merge remote-tracking branch 'refs/remotes/origin/master' into global-output-131

Conflicts:
	docs/source/news.rst
Update cross-process documentation and examples to help identify which
messages come from which process.
Merge pull request #133 from ClusterHQ/global-output-131

Global field output.

Fixes #131.
Merge remote-tracking branch 'refs/remotes/origin/master' into sort-order-129
Revert sketch of changes, as prelude to TDD implementation.
Update Action and Message implementations to know about new TaskLevel
class, sufficiently to make test_action pass.
Fix most remaining tests, and on relevant piece of code, to know about
new TaskLevel class.