Create ActiveEdgeTriplet to expose activeness in GraphX

GraphX's mapReduceTriplets operator supports filtering edges by the activeness of adjacent vertices, but currently does not provide that information to the user map function. If the active direction is `In`, `Out`, or `Both`, the user can infer which vertex was active, but if it is `Either`, the map function has no way of knowing.

This PR exposes vertex activeness on edge triplets by subclassing EdgeTriplet to provide `srcActive` and `dstActive` properties. It also cleans up the GraphSuite mapReduceTriplets test slightly.

This was a feature request by @semihsalihoglu.