LuaRegistry

pimpl class, LuaRegistry, to keep registered functions / objects in c
memory
moved LuaStackGuard into the Helpers folder
fixed error in _getLuaTable (could accept a negative index)
rearranged LuaCaller implementation
new function added to LuaCaller that returns number of returned values
moved impl::BaseFunctionWrapper into function_impl.h
various updates/changes in several files