Support std::future<void> with thread_pool::enqueue

[DeveloperPaul123]

Currently it's not possible to do the following:

dp::thread_pool pool;
auto value = 8;
auto future = pool.enqueue([](int& x) { x *= 2; }, std::ref(value));
future.wait();

dp::thread_pool should support this use case.

With the upcoming std::future updates and extensions (i.e. std::when_all, std::when_any) we may not need #27. Though, having something built in to the class would save on memory usage since users would presumably have to store std::future<void> in a list of some sort (i.e. std::vector<>).