PartitionStrategy hides the specified partition key from view #397

Hidden during iterating properties, keys, etc.
Make construction for ReadOnlyGraphStrategy consistent. #417
Minor renaming in ReadOnly/IdentityStrategy. #417
Changed method of construction for PartitionStrategy to builder pattern #417
Add builder pattern for SequenceStrategy. #417

Still more work here though - not all tests can currently pass with this model.
Dropped GraphStrategy parameter from GraphFactory.open()

The perferred method is to use Graph.strategy() now.
Add builder for SubgraphStrategy #417

PartitionStrategy no longer extends SubgraphStrategy.  This allowed the build() method to stay consistent and set up for major changes to PartitionStrategy in #419
Change scope of SubgraphStrategy constructor #417
Added composingStrategy parameter to all GraphStrategy methods.

Now a GraphStrategy implementation can get access to the GraphStrategy that is composing the function returned from it.  This is mostly useful for nested GraphStrategy implementations where a dynamically constructed GraphStrategy function from the parent is required.