Use a single random object injected during core driver initialization.
Use {@inheritDoc} in core drivers.
Setting method visibility for the expandEntityFields() method.
Declare class variables.

- Use `version` throughout instead of `drupalVersion`.
Random generator is public.
Merge pull request #59 from jhedstrom/code-cleanup

Code cleanup
Docblocks for class variables.
Split driver loaders into private methods.
Removing unused variables.
Clarifying docblocks.