The NTPClient method update() has been changed to only return "TRUE" after a successful update. The following conditions return "FALSE":
- Interval duration requirement not met
- forceUpdate() times out
I added two functions:
```cpp
setTimeOffset(int timeOffset)
```
Allows you to set the offset at a later stage (e.g. upon timezone
changes or change from summertime to standard time)
```cpp
setUpdateInterval(int updateInterval)
```
Allows to set the interval at a later point. This was helpful when not
setting an offset at initialization and e.g. for other sync times during
special occasions.
