luaengine: add romname() and pause()/unpause()

Hi,
this is a really small PR to add the following methods to MAME lua API:
 * emu.romname()
 * emu.pause()
 * emu.unpause()

These are mostly being added here in order to start closing the gap with [mame-rr lua interface](https://code.google.com/p/mame-rr/wiki/LuaScriptingFunctions).

Signed-off-by: Luca Bruno <lucab@debian.org>