Private hash calculation of a VersionVertex.

Implementation of a private hash (based on SHA-1) calculation of a VersionedVertex.
A private vertex hash is a calculation of the vertex properties without
taking into consideration the vertex's edges.

This commit also contains:
- A rename of VersionedVertex.version -> forVersion
- A rename of VersionedGraph.isPropertyInternal to isInternalProperty
- Tests for the private hash calculation.