Allow building Tenacity without Conan

Conan can still be used, but it is fully optional. This should help packagers
a lot.

Signed-off-by: Edgar <Edgar@AnotherFoxGuy.com>

lib-src/ThreadPool/README.md

@@ -0,0 +1,17 @@
+ThreadPool
+==========
+
+A simple C++11 Thread Pool implementation.
+
+Basic usage:
+```c++
+// create thread pool with 4 worker threads
+ThreadPool pool(4);
+
+// enqueue and store future
+auto result = pool.enqueue([](int answer) { return answer; }, 42);
+
+// get result from future
+std::cout << result.get() << std::endl;
+
+```