luaengine: add access to devices and address spaces

This commit maps some more classes (device_t and address_space)
to retrieve devices and address spaces out of current running
machine.
Proxy methods are provided to enumerate/access devices and
address spaces, and to read memory content.

Signed-off-by: Luca Bruno <lucab@debian.org>
Merge pull request #89 from lucab/lucab/mame-lua/mem-v2

luaengine: add memory-reading capabilities [Luca Bruno]