Allow dereferencing of Agent values while updating

ensure only one update is running
Make Agent work with ImmediateExecutor
whitespace
Add support for await as in Clojure
Extract OneByOne behavior from Agent

it can be also later used for Actors
Run Actors in pool do not spawn Thread per Actor
Merge pull request #66 from pitr-ch/actor

Actor improvements
Added pitr-ch as a contributer in the README.
Agent#await does not alter the value
Support sending non/blocking fns to Agent
whitespace
Merge pull request #71 from pitr-ch/actor

Support `send` and `send-off` methods for Agent
Refactored TimerTask specs.
Merge pull request #75 from jdantonio/refactor/timer-task-specs

Refactored TimerTask specs.
Prerelease 0.6.0.pre.2
Fixed brittle RubyThreadPoolExecutor spec.
IVar: move MultipleAssignmentError into the class.
IVar: documentation.
MVar: documentation.
TVar: documentation.