A basic Repository implementation

There are a few things remaining here:
  - rdf-spec doesn't seem to work with the latest versions of RSpec,
  so *all* of this requires some further verification.
  - BNodes are probably very badly handled, here. Two statements that
  have different bnode subjects, but are otherwise identical will likely
  both be deleted. This is also the case in SPARQL::Client::Repository.
  - There are some issues with Marmotta's SPARQL endpoint. I'm opening
  issues on their JIRA and on this project.
Adding basic tests (and pending for known issues)
Creating a README.md file