[FLINK-1032] Rework support for POJO types in the Java API

With this change, users can use string-based expressions to select keys on pojos and tuples.

POJOs (=plain old java object) are bean-style java classes that have public getter/setter for each non-public field and a constructor without arguments.
This change allows in particular to select nested pojos.
To illustrate it, imagine you have the following POJO
```java
	public static class PojoContainingTupleAndWritable {
		public int someInt;
		public String someString;
		public IntWritable hadoopFan;
		public Tuple2<Long, Long> theTuple;
		public FromTuple fromTuple;
		public PojoContainingTupleAndWritable() {}
		public PojoContainingTupleAndWritable(int i, long l1, long l2) {
			hadoopFan = new IntWritable(i);
			someInt = i;
			theTuple = new Tuple2<Long, Long>(l1, l2);
		}
	}
	// nested element
	public static class FromTuple extends Tuple3<String, String, Long> {
		private static final long serialVersionUID = 1L;
		public int special;
	}
```

You can now use these kinds of string-based expressions:
```java
// group by "hadoopFan" 
DataSet<Integer> reduceDs = ds.groupBy("hadoopFan")
// include all fields of the Tuple2
DataSet<Integer> reduceDs = ds.groupBy("hadoopFan", "theTuple.*")
// use full pojo
DataSet<Integer> reduceDs = ds.groupBy("*")
// nested pojo access
DataSet<Integer> reduceDs = ds.groupBy("fromTuple.special")
```
Note that `FromTuple` is handled as a pojo, not a tuple.

The pull request contains changes from @aljoscha that I've included into my own commit.


There are the following things missing:
 - Documentation
 - One TypeExtractor test case is disabled
 - the partition operator has not support for the expressions yet. Ideally its just an issue of changing the interfaces and adding a test
 - Somehow, operator chaining (mapper and combiner) does not work with pojos. I'll look into this.

Does this indicate that you found and fixed another bug on the way (screwing up fields orders through the sorting?)