AsyncScheduler: implement _poll
AsyncScheduler: poll in _schedule_tasks

Triggers cleanup and exit listeners if there's nothing left to do.
ManifestTask: add missing signatures

If the existing Manifest already has the correct content, but it is
not signed, then sign it if appropriate.
ManifestScheduler: handle InvalidDependString
repoman: avoid Manifest double signature
ebuild.sh: comment on noauto behavior
test_ipc_daemon: implement internal SleepProcess

Emulate the sleep command, in order to ensure a consistent return code
when it is killed by SIGTERM (see bug #437180).
TaskScheduler: inherit AsyncScheduler

This allows the QueueScheduler class to be eliminated.
AsyncScheduler: remove useless __slots__

It's not allowed for AsyncScheduler to inherit from two classes that
define __slots__, so it useless to define __slots__ for AsyncScheduler
and subclasses.
PollScheduler: remove self._jobs

This is variable is only needed by the Scheduler class.
PollScheduler: remove register/unregister methods

These methods were aliases for the EventLoop io_add_watch and
source_remove methods. Migrating to the EventLoop method names allows
an EventLoop instance to substitute for a PollScheduler inside
subclasses of AbstractPollTask.
Substitute EventLoop for PollScheduler.

EventLoop suffices for all of these cases. EventLoop(main=False) is
used for thread safety where API consumers may be using threads.
ManifestScheduler: tweak _task_exit output order

This ensures that errors are logged before new jobs are scheduled.
MetadataRegen: inherit AsyncScheduler
PollScheduler: move _main_loop to Scheduler
Run tests with Python 3.4.
best_version() and has_version(): Improve die() messages.

- Print different die() message for invalid atoms.
- Mention ebuild-ipc instead of portageq in other die() messages when
  ebuild-ipc is actually used.
action_uninstall: use PollScheduler not Scheduler

The PollScheduler class suffices here, if we just add a small amount of
logic to calculate the _background attribute from the emerge opts.
PollScheduler: split out SchedulerInterface
Substitute SchedulerInterface for PollScheduler.

SchedulerInterface suffices for all of these cases.
EventLoop(main=False) is used for thread safety where
API consumers may be using threads.