v2.1

• Merged latest changes from PRISM (up to v4.5 and slightly beyond),
  including directory rearrangement (some files moved to top-level)
• Examples and tests tidied up (now in prism-examples/smgs directory
  and prism-tests/functionality/verify/{smgs,smgs-multi})
• Simplified dependencies on the PPL library
  (in particular, local copies are stored, so binary releases are self-contained)
• Version number of PRISM-games properly reported/displayed
• Some compositional functionality moved into separate branch for now
• Various bugfixes