inDegrees and getDegrees implemented and tested
subgraph(vertexFilter) and subgraph(edgeFilter) implemented and tested
outDegrees with coGroup
filterOnVertices and filterOnEdges implemented and tested
inDegree, outDegree, getDegrees implemented and tested
Merge pull request #22 from andralungu/subgraph

subgraph(vertexFilter) and subgraph(edgeFilter) implemented and tested
cleared VertexKeyWith one class definintion
Fixed merge conflicts for rebase
Merge pull request #21 from andralungu/degrees

inDegrees and getDegrees implemented and tested
replace fromCollection with fromElements; remove unused import