Add SimpleMutex class and platform specific mutex function

Add SimpleMutex class and platform specific mutex function and SimpleMutex unit test
I agree. But I do think that, if one contributor has created or strongly influenced the genesis of a file (as is the case here), it would be nice if he had a mention in that files (c), after Michael Feathers, James and you, as in
```
 * Copyright (c) 2007, Michael Feathers, James Grenning, Bas Vodde and Chen YewMing
```
If you prefer the to be all exactly the same, then I suppose I'll have to remove my own name in a few places ;-)