[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.

Should get rid of this TODO.