adds comments about confusing test mode nomenclature mis-match…

[jasonfb]

between gateway code and Auth .Net; also adds ActiveSupport deprecation notice if you fail to have your preferred server set to 'live' or 'test'; defaults to 'live' gateway with test-mode set to false (also set test mode to false; unlike other gateways, it does not determine if the test server is used)

[jasonfb]

@Senjai

since test_mode is defined on the base objects, I don't see anyway to 'take it off' these two objects (where it really shouldn't exist). As a compromise, I've made it default to false with comments as warning messages that its meaning is different for Authorize .Net than from the other payment gateways and to warn people to make sure their 'preferred server' is set to either 'live' or 'test'

[jasonfb]

@Senjai -- since I can't remove it, I've added it with :default => false to one of the objects (one already had it) so that at least it is consistent. Although not perfect, I think this a reasonable way forward.