Fix MSVS 2013 warning about a condition being always true.

Use "for(;;)" for the infinite loop to avoid the warning at /W4 warning level.

as suggested by https://github.com/n00btime
return type deduction now uses std::result_of
which allows return type deduction for pointer
to members (and looks cleaner anyway).

This change allows the "library" to stay
header only. This might not be best practice
but helps keeping the more educational code
compact.

As suggested by daniel-j-h tasks are now taken by
universal reference and forwarded with std::forward.
Also enqueue takes a variadic argument list and binds
the arguments to the task.

Replaced deque by a queue (which by default uses a deque).
Simplified the task packaging as suggested by Ang3lus in issue #4.
The destructor now waits for the queue to empty but throws
a std::runtime_error if enqueue is called on the stopped pool
(see issue #3).