Add PojoTypeInformation, PojoComparator and Expressions for Keys

This enables code like:
```java
class WC {
    public String word;
    public int count;
}
...
DataSet<WC> result = tokenized
		.groupBy("word")
		.reduce(new ReduceFunction<WC>() {
			public WC reduce(WC value1, WC value2) {
				return new WC(value1.word, value1.count + value2.count);
			}
		});
```