Allow programs to override this value

I'm not positive the actual limits of an Uno or a Mega for this library, but I know a Mega can do more than 10 timers and I need more than 10 for my project - this compiler switch allows programs to
```
#define MAX_NUMBER_OF_EVENTS (45)
#include "Timer.h"
```
And have a number different than 10, or
```
#include "Timer.h"
```
And go with the default of 10