Move guard-rspec out of CI gems
Spec for thread count during pool execution
Use the timers gem
Bump timers gem dependency
Don't proxy the #class method

I've gone back and forth on this, and now I feel like proxying #class
and hiding the identity of the PoolManager is a bad idea. It's confusing
to those who don't realize that there's a PoolManager proxy between them
and the workers.
Prevent SystemEvents from leaking out of #receive
Update CHANGES
Bump timers dependency to 1.0.0.pre3
Bump timers dependency
Use Actor.all in pool spec
Use timers 1.0.0
Bump version to 0.11.1
Log the cause of aborted async calls

This was previously logging the AbortError, which doesn't give us useful
information.
SystemEvents are no longer exceptions

SystemEvents aren't necessarily errors or even exceptional, they're just
high priority messages. Once upon a time when they were used exclusively
for exit messages (caused by errors) it kind of made sense for them to
be exceptions, but they have grown in scope and usefulness and really
should just be objects.
Eliminate Celluloid::Mailbox#system_event

Celluloid::Mailbox automatically detects system event messages and
treats them as being higher priority
Visual cleanup
Link with SystemEvents

This introduces the LinkingRequest system event which improves system
robustness and allows linking requests to be handled seamlessly both in
exclusive mode and when using #receive

In addition, link/unlink now use exclusive mode, making them atomic and
transactional. This makes linking semantics easier to reason about.
Unidirectional linking with #monitor and #unmonitor
Handle system events properly when exclusive
Don't process system messages during linking

Processing system messages may kick off other events that can interfere
with the linking process. Instead, system messages are queued and
processed after linking operations are complete.