[FEATURE] added a concurrency-safe queue 

Features:
 - the queue can be fixed-size or grow dynamically
 - it supports multiple readers and writers
 - it supports C++11 move so that (in theory) elements w/o copy c'tors can be enqueued, e.g. resources like threads, semaphores
In C++11 mutable means "bitwise const or internally synchronized". Here it is the latter and Herb Sutter even used a Mutex like this ReadWriteLock as an example for mutable.