Add a test to distinguish between group_node_label and group_label.

When we run this test on a disco cluster with more than one node, we can
see the improvements of the group_node_lable over group_label.  With
group_node_label, for each intermediate step, one task is created on each
node and the inputs are processed locally.  With the later, only one
task is created on one of the nodes and all of the input is shipped to
that node.  If the size of the input files is large enough, we can see
significant improvements.
Add a test for using raw input data in the pipeline version.
Add a test to chain a set of stages to join two tables.

This test might fit better as an example than a test.  It examines the
interactions of the different stages to efficiently join two tables.
Merge pull request #9 from pooya/shayan/group_test

Add a test to distinguish between group_node_label and group_label.