Add a executor backed conductor and have existing impl. use it

This adds a executor backed job dispatching base class
and has the existing blocking executor use it by running jobs
and dispatching jobs into a sync executor. It also allows
for dispatching jobs into a thread executor, or other executor
via a new '_executor_factory' method that can generate
executors (it can be overriden in the non-blocking executor
to provide your own executors instances).

This does alter the behavior in that now that jobs are dispatched
into an executor we no longer can immediatly know if a job was
dispatched and raised an exception or whether it will raise an
exception in the future, so we now alter the 'local_dispatched'
to just be a boolean that is used to determine if any dispatches
happened (failure or not).

Change-Id: I485770e8f4c85d3833892a453c9fb5168d8f0407

doc/source/conductors.rst

@@ -9,7 +9,7 @@ Conductors
 Overview
 ========
 
-Conductors in TaskFlow provide a mechanism that unifies the various TaskFlow
+Conductors provide a mechanism that unifies the various
 concepts under a single easy to use (as plug-and-play as we can make it)
 construct.
 
@@ -66,6 +66,7 @@ Interfaces
 
 .. automodule:: taskflow.conductors.base
 .. automodule:: taskflow.conductors.backends
+.. automodule:: taskflow.conductors.backends.impl_executor
 
 Implementations
 ===============
@@ -75,12 +76,19 @@ Blocking
 
 .. automodule:: taskflow.conductors.backends.impl_blocking
 
+Non-blocking
+------------
+
+.. automodule:: taskflow.conductors.backends.impl_nonblocking
+
 Hierarchy
 =========
 
 .. inheritance-diagram::
     taskflow.conductors.base
     taskflow.conductors.backends.impl_blocking
+    taskflow.conductors.backends.impl_nonblocking
+    taskflow.conductors.backends.impl_executor
     :parts: 1
 
 .. _musical conductors: http://en.wikipedia.org/wiki/Conducting