A single line typedef, adding ofScopedLock (aka Poco::FastMutex::ScopedLock)

I have been using `Poco::FastMutex::ScopedLock` extensively lately and have noticed others doing the same.  I propose adding this single line typedef to make lives easier and code cleaner.

For those who haven't experienced the wonders of scoped locks, it is a technique that helps ensure that mutexes get released/unlocked when they go out of scope, preventing deadlocks (i.e. the destruction of a scoped lock releases the mutex) resulting from forgetting to release / unlock your mutexes after use.

For instance:

```c++
void myApp::callAccessingSharedResources() {
    myMutex.lock();
    mySharedResource.modify();
    myMutex.unlock(); // you can't forget this!
}
```
could be replaced with:

```c++
void myApp::callAccessingSharedResources() {
    ofScopedLock lock(myMutex); // myMutex.lock() called upon construction of lock
    mySharedResource.modify();
   // as lock goes out of scope, its destructor is called
   // inside of its destructor calls myMutex.unlock()
}
```