Fix variable type

Following the README doesn't work unless `pool` is replaced with `financial`.
Merge pull request #4 from zph/master

Fix variable typo
Event#set? changed to ensure visibility
Event#wait policy changed with a ConditionVariable
Event#set and Event#reset changed to support the new policy
Event general refactor
added: ImmediateExecutor, a simple executor where tasks are executed by thread that posted them
added: ImmediateExecutor specs
future_spec, context fulfillment, changed to use ImmediateExecutor, so it can run faster
Merge pull request #5 from mighe/master

Event potential concurrency issues
Added contributors to the README
fixed: Dereferenceable mutex race condition
Merge pull request #6 from mighe/master

Dereferenceable mutex race condition