rrdf: Release 2.2.1

This release has the following changes:

• adds (unit) testing with the testthat package
• adds a tripleCount(store) method
• some documentation improvements
  • typo in package description
  • now lists all formats in asString()
• add.data.triple() and load.rdf() should now stop() when exceptions happen (closes #10)
• add.data.triple() now checks the data type given
• sparql.remote() now discusses bif:contains (closes #32)
• now uses missing() instead of is.null() for input checking (addresses #34)