Introduce a ScopedTable class to cleanup Lua tables at object destruction

In some cases, it is handier to automagically cleanup the stack when the
C++ wrapper is destroyed, for instance when operating on a LuaRef. To
have that behaviour, the LuaTable(LuaRef) constructor has been removed,
replaced by a ScopedTable(LuaRef) one.
LuaTable: remove useless virtual qualifier for the destructor
Merge remote-tracking branch 'laarmen/scopedtable'