Make it safe to turn off/on profiling timer in code

Certain code (e.g., a `ccall` to `fork`) seems to cause trouble for the profiler. This allows one to turn off the profiling timer, e.g., 
```
    profiling = Profile.is_running()
    Profile.stop_timer()
    # code that shouldn't be profiled
    if profiling
        Profile.start_timer()
    end
```

@vtjnash, @loladiro.