Do not shutdown the mailbox when an IOError occurs in EventedMailbox

This is redundant as the exception which is raised will shutdown the
mailbox immediately afterwards.
Only shutdown the mailbox when killing the Actor if it is alive
Celluloid::Properties

A "property" macro for adding declarative properties to a class which are
inherited but overridable in subclasses. Sort of like what class variables
always should've been.

Just an idea for now, but this can be used to replace things like
"mailbox_class" and "execute_block_on_receiver"
Use Celluloid::Properties for all actor class properties

This results in some minor API changes, specifically mailbox.max_size as a class
attribute, which is replaced in this commit as mailbox_size

I know there are plans to change these, but this largely preserves the existing
API with a few minor changes.
Allow supervision group run/run! inits to use a private actor registry
Merge pull request #287 from celluloid/properties

Celluloid::Properties
Improve thread management

This asserts that the internal thread pool is shutdown before booting
and also tracks all threads while busy.

Use Celluloid.internal_pool for all code instead of Thread.list
Cleaned up variable names and line spacing.
Merge pull request #279 from celluloid/safe-threads

Improve thread management
Merge pull request #293 from ehogberg/supervision_group_private_registry

Allow supervision group run/run! inits to use a private actor registry
Add the InternalPool max_idle accessor back
Allow TaskThreads to be created while Celluloid shutdown
Indent task list and backtraces further

This more clearly indicates that they are associated with the preceding actor
Delete annoying log message

Everyone seems to hate it. Might as well delete it I guess :(
Ignore tmp/
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.
setup_properties is unnecessary
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