Renamed major components of Oclgrind.

Source directories:
  src/spirsim         -> src/core
  src/oclgrind        -> src/runtime
  src/oclgrind-kernel -> src/kernel

Libraries:
  spirsim      -> oclgrind
  oclgrind     -> oclgrind-rt
  oclgrind-icd -> oclgrind-rt-icd
Updated README with library rename changes.