Add backtraces to "Dangerously suspending task" messages

Right now there's no way to determine where dangerous suspends are occurring.
Including backtraces in the warning message alows us to track down where these
problems are coming from.
pluralize actor(s)
Merge pull request #297 from karlfreeman/pluralize-actors

Pluralize actor(s)
Merge pull request #296 from celluloid/dangerous-suspend-backtraces

Add backtraces to "Dangerously suspending task" messages
use double method to create dummy object. mock & stub methods are deprecated.
Merge pull request #299 from unakatsuo/rspec-mock-deprecation

Supress DEPRECATION messages while running spec.
Fix InternalPool to support third-party threads

See issue #298.
InternalPool cleans up dead threads
[BUG] ActorProxy doesn't respond_to respond_to? correctly
Merge pull request #302 from celluloid/respond_to_broken

[BUG] ActorProxy doesn't respond_to respond_to? correctly
Merge pull request #298 from celluloid/third-party-threads

InternalPool does not support non-Celluloid threads being created
Revert "[BUG] ActorProxy doesn't respond_to respond_to? correctly"

This reverts commit 45beb53d35f4fc8da7df9e7d9d84f04d26e5eb4b.

This broke the entire test suite so none of the other tests even run o_O

/cc @benlangfeld
Only log dangerous suspends if $CELLULOID_DEBUG is set

These warnings aren't helpful and are way too annoying for average users.

We should work on fixing the internal sources of these warnings before we expose
them to average users of the framework.

At the very least we need a way of acknowledging them and hiding them in
situations where they're irrelevant.
[BUG] ActorProxy doesn't respond_to respond_to? correctly
Merge pull request #303 from celluloid/respond_to_broken

[BUG] ActorProxy doesn't respond_to respond_to? correctly
Always enable  when celluloid/rspec is included
Revert "[BUG] ActorProxy doesn't respond_to respond_to? correctly" #303

This reverts commit e4c2ade6c204f2a3eb4bc070cd18de68e361aa8e.
__class__ for all AbstractProxy descendants

This provides access to the current proxy class for classes which inherit from
Celluloid::AbstractProxy. This can be used for methods that need reflection.
Implement #respond_to? on SyncProxy

Check the instance methods of the proxy class first when responding to
respond_to?

If that fails, ask the actor if it responds to the given method.
[BUG] ActorProxy doesn't respond_to respond_to? correctly