Add Shortest-path computations to graphx.lib with unit tests.
Revert "Add Shortest-path computations to graphx.lib with unit tests."

This reverts commit 7496d6b7181ef9eecfdee5c84f154696302821df.
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/spark
Merge branch 'master' of https://github.com/apache/spark