The Jacob Martin Band is electrifying. Possessing mainstream country-pop appeal blended smoothly together with a rock and blues-infected charm, the group -- comprised of Jacob Martin (vocals/rhythm guitar), Josh Carlson (drums/background vocals), Jeff Coggins (lead guitar) and Mike Guy (bass/background vocals) -- took the stage this week at 12th and Porter in Nashville to perform a short 45-minute set. After witnessing a charged, dynamic set, you would never have guessed that their start began from a simple Craigslist ad a…