get list of all era ids and allow request for config of era by id

For SigurdFD:

Not extensively tested, but it is a niche feature so I hope SigurdFD will let us know if something is bugged here.

The eras list, wesnoth.game_config.eras, seems to work perfectly.
The wesnoth.get_era function is not extensively tested, as this would be somewhat hard, but here is some sample output, from booting any scenario with --debug, and typing
":lua wesnoth.message(wesnoth.debug(wesnoth.get_era("WC_era")))"
http://i.imgur.com/EAKan4p.png

WC_era = world conquest era