Refactor Push() to remove need for int argument

[zyvitski]

From using the CTPL library I have noticed that the fact that push() requires the first argument of the function being pushed makes using the library difficult at times.

In a way it forces anyone using the library to force their interface to conform to the need for that int parameter. This can be achieved by either conforming your interface to work with the pool or by wrapping your target function with another function that adds the leading int parameter.

I believe that it would make the library more accessible, transparent and user friendly if an overload of push() was provided that allowed you to push a function onto the pool without the need for the leading int parameter.

I would be willing to work up the changes and add them as a pull request if desired, but I wanted to run the idea by first.