[FLINK-1040] Make type() call in projections optional (or remove it)

The original version of projection requires a type() to specify the types of output dataset, e.g., dataset.project(0).type(string.class)). Now, I implement a new interface projection() which does not require the type() call, i.e., dataset.projection(0). Similarly, I modify the operation of JoinOperator and CrossOperator. In particular, some changes in ProjectorOperator.java, JoinOperator.java and CrossOperator.java are generated by TupleGenerator.java.

I add more testing cases here. In addition, I rewrite a new example of web log analysis by the projection() API.
This example still contains calls to `types(Class<?>...)`. Is this on purpose? I think we should have the call out of all examples. Having them in the tests is OK though. :)