Add landmark-based Shortest Path algorithm to graphx.lib

This is a modified version of apache/spark#10.

Author: Ankur Dave <ankurdave@gmail.com>
Author: Andres Perez <andres@tresata.com>

Closes #933 from ankurdave/shortestpaths and squashes the following commits:

03a103c [Ankur Dave] Style fixes
7a1ff48 [Ankur Dave] Improve ShortestPaths documentation
d75c8fc [Ankur Dave] Remove unnecessary VD type param, and pass through ED
d983fb4 [Ankur Dave] Fix style errors
60ed8e6 [Andres Perez] Add Shortest-path computations to graphx.lib with unit tests.

(cherry picked from commit 9535f4045daf46b084761d7f15f63dc6c2a543dd)
Signed-off-by: Reynold Xin <rxin@apache.org>