Allow to use timeout only on #post_off

- post should not be blocking or take long
- defaults to no timeout (no extra threads)
[Do not merge, RFC]Add dataflow!

With futures, you can call ```value!``` if you want exceptions to bubble up. It'd be rather nice if you could do the same thing with dataflow.

This is the simplest possible solution, if you collectively think this is a worthwhile feature I'll polish it up, add tests, etc.
Fix brittle test
tests for raising errors
remove redundant test here
cleanup
De-duplicate code
Merge pull request #131 from mastfish/patch-1

Add dataflow! to match value!
Merged PR #69: Do not create another thread for each post call to Agent
Merge branch 'pitr-ch-agent'
Updated README.
Fix unbalanced parenthesis in README
Merge pull request #135 from obrok/patch-1

Fix unbalanced parenthesis in README
Do not create un-GC-able symbols when creating ThreadLocalVars

GC all data for a given thread local var when the var is GCed
Added ruby critic rake task.
Use global_operation_pool for jobs which may block
Merge pull request #137 from pitr-ch/pools

Use global_operation_pool for jobs which may block
Temporarily adding Rbx to Travis allowed failure list due to problem with the Travis service.
Merge pull request #136 from ruby-concurrency/thread-local-var

Do not create un-GC-able symbols when creating ThreadLocalVars
Merge remote-tracking branch 'upstream/master' into actress

* upstream/master: (13 commits)
  Temporarily adding Rbx to Travis allowed failure list due to problem with the Travis service.
  ...

Conflicts:
	lib/concurrent/atomic/thread_local_var.rb