mapVertices allows changing the value type
remove old hackathon test file
separate test for mapVertices
Merge branch 'master' of https://github.com/project-flink/flink-graph into mapVertices
Merge pull request #7 from project-flink/mapVertices

mapVertices implementation
fix conflicts with current master
empty test todos for implemented operations
fix count to work with Datasets of tuples, added test for numberOfVertices, numberOfEdges
static inner classes for getVertexIds, getEdgeIds, isWeaklyConnected, tests for former 2
isWeakly connected corrections and test