Support TestGraph's edge value.

Fixes #86

Input format: [vertexId]:[vertexValue] [neighborId1:edgeValue1] [neighborId2:edgeValue2] ....

Sample input: 
1:1.0 2:2.2 3:3.3
2:2.0 3:3.3
3:3.0 2:2.2

If all vertexValue or edgeValue are not provided, they are assumed as null, i.e., this input is also accepted with edgeValue as null:
1:1.0 2 3
2:2.0 3
3:3.0 2

When some vertexValue/edgeValue are provided and some are not provided, the missing ones are assumed to 0. That is,
1:1.0 2:2.2 3
2:2.0 3
3:3.0 2
is equivalent to
1:1.0 2:2.2 3:0
2:2.0 3:0
3:3.0 2:0