Added mapEdges
Added mapEdges tests
Removed unnecessary edgeKeyType field
Updated the Graph constructor
Added mapEdges to the tested methods
Merge pull request #6 from balidani/master

mapEdges implemented
fromCollection(vertices, edges) implemented and tested
Merge pull request #13 from andralungu/fromCollection

fromCollection(vertices, edges) implemented and tested
count to consider 0 elements
fix merge conflicts with current master
got rid of ExecutionEnvironment in Graph