Added Duration class

Timespans or durations are often hard to document and a developer might not always know which unit a method parameter expects (usually it's milliseconds, but in this case we also have Minecraft ticks). 

In order to simplify this, I created a simple Duration class that holds an internal value in the milliseconds unit and converts to Minecraft ticks, seconds, minutes and hours. It can also do addition and subtraction.

Code example:
```java
Duration d = Duration.minutes(10).add(Duration.seconds(30));
System.out.println(d.getMillis()); // should print "630000"
```