Always create a Task regardless of the Actors exclusives

This change means that we need a new concept to distinguish the old
behavior which did not create a Task, but instead gave users direct
exclusive access to the actor Thread.
Remove deprecated finalizer code
Merge pull request #281 from celluloid/exclusive-task

Always create a Task regardless of the Actors exclusives
Merge pull request #256 from celluloid/remove-deprecated-finalizer

Remove deprecated finalizer code
Improve the structure of StackDump
Merge pull request #286 from celluloid/cleaner-stackdump

Improve the structure of StackDump
JRuby build shouldn't be allowed to fail

We're past the fiber breakage in JRuby 1.7.3
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