…rn value

Add the ability to enqueue a task that does not require monitoring a
return value.  std::future<T> on destruction is expected to wait() for a
thread to complete before completing destruction (btw, MSVS2103
std::future/std::ansync does not behave according to C++ Standard).  If
a temporary future is created (not captured), the calling thread should
block waiting for the thread to exit and complete the promise.  An
alternate enqueueAndDetach method was added to allow for invocations
that don't require the capturing of the std::future.

Typo in comment

Break the source into header and implementation, make the template
definition more readable by changing F to Callable

Rename ThreadPool.h to ThreadPool.hpp

Comment update

Use typename instead of class

Provide a single exit point for the worker thread

Use lock_t instead of Lock

enqueAndDetach is not needed as std::future wait is not an issues on
it's destruction if it is not created from std::async.