Very minor spec tidy ups

Used `subject` where possible.
Slight naming fixes.
Removed unnecessary `subject` from nested contexts.
Implement super simple timers in tasks

Timeout when a task is suspended for more than a certain time.
Don't bother naming a block if we're not passing it anywhere
Better backtrace on timeouts
Reverted changes to the explicit `subject`.
Add role to Celluloid::Thread to distinguish thread ownership
Use ThreadHandle throughout
Merge pull request #242 from benlovell/spec-tidy-ups

Very minor spec tidy ups
Merge pull request #244 from celluloid/fix-stack-dump

Actors which use TaskFiber and have a current runnning Task do not appear in StackDump
Merge pull request #243 from celluloid/timeout

Implement super simple timeouts in tasks