Added sleep helper function.

Fast-fix for #35.  Alternative would be: `boost::this_thread::sleep_for(const chrono::duration<Rep, Period>& rel_time)`, as in

```
void sleep(long millis){
    boost::chrono::duration<long, boost::milli> milliseconds;
    milliseconds toSleep(millis);
    boost::this_thread::sleep_for(&toSleep);
}
```