lua-bindings: added open_lualib() for on-demand loading of Lua libs.

lua-bindings: added open_lualib() for on-demand loading of Lua libs.

Use open_lualib to load standard Lua libraries as needed. You can call
it with an argument list consisting of the names of the libraries you
want to load (IOW to luaopen_*()). Eg.
  murphy.get():open_lualib('math', 'string', 'io')
will try to luaopen_math(), luaopen_string(), and luaopen_io().
The available libraries are 'math', 'string', 'io', 'os', 'table',
'debug', 'package', and 'base'.