Revert "More forgiving thread local hacks"

This reverts commit c200845061447c010ca7e6e8069fe400c56a0427.
Revert "Actor-local Celluloid::Thread#[] support"

This reverts commit 395a6dff992a16609baa94331b50c4848678967d.

Conflicts:
	lib/celluloid/thread.rb
Reapply custom Thread methods
Merge pull request #268 from celluloid/revert-actor-locals

Revert problematic Actor-locals
Bump version to 0.15.0.pre
Remove unused Task#name accessor
Use a hash for Task metadata
Log discarded messages inside Actor and on Mailbox shutdown. Fixes #277