Safer handling of the Lua stack (GH-188)

Some places don't lock the LuaRuntime before fiddling with the Lua stack, and many places don't ensure extra slots either.
This PR also involves other changes:

*  Add preconditions and postconditions to important functions/methods
*  Change behaviour _LuaObject.__str__
   *   If __tostring doesn't exist, use _LuaObject.__repr__
   *   If __tostring returns non-string value, raise a TypeError
   *   If __tostring raises an error, raise a LuaError
*  lua_pushglobaltable instead of pushing _G, which may not point to the global table.
*  Make lock_runtime return boolean and not raise error directly
*  Use LUA_NOREF and LUA_REFNIL constants for effective handling of Lua references
*  Add important assertions (checks if LuaRuntime._state is not NULL, and if _LuaObject._runtime is None, etc...)