typedef InterpreterCache maps.
typedef WorkItem builtin map and prefix list.
typedef WorkItem variable map.
printf using getOperand() instead of resolveConsrExpr.
Constant expressions now cached.
Initial plugin work.

Base plugin class declared in plugins/Plugin.h. Device loads plugins
during creation. Plugins override callback functions required, which
are fired via Device.

Refactored instruction histogram functionality into an
InstructionCounter plugin.
Install Plugin.h.
Moved Plugin.h to src/core.
Don't install InstructionCounter.h with CMake.
Fixed generation of oclgrind script for CMake.
Simplified EXTRACT_OFFSET macro in Memory.cpp.