Initial implementation of virtual nodes
Fixes for virtual nodes and initial perf test class

Introduced a dependency on an extension to dotNetRDF to enable VNodes to
perform better. So this feature branch will remain open until DNR is
officially updated.
Start of support for custom and composite keys
Can now include the key of other entities in composite
Merge branch 'release-1.6.2' into develop
Refactoring to implement custom identity checking
Change to using a singleton for EntityMappingStore
Removed build of deleted SPARQL server sample
Refactored identity mapping info

Now consistently uses the suffix string as the value exposed by the
identity property. A few unit tests remain to be updated.
Fixed EF tests
Added unit tests for generated keys
Implemented preconditions for key constraints
Merge branch 'feature/entity_keys' into develop
Fixes for portable build
Fixed mobile build
Added documentation about generated keys
Added documentation on new transaction API
Added whats new on develop branch
Jobs now get a default label

Fixes issue #93
Ensure service returns the full job status

Fix for issue #101