(Twitter's Songbirds. photo: Carla Bova) It began like any other concert: The house lights dimmed. A hush came over the audience before it erupted into excited applause. A foggy haze wafted from stage right, enveloping four standing mics. Spotlights brightened to beckon the opening act on stage. It was a setup for rock stars, and the 13 singers who made their entrance seemed right at home. They were the a cappella stars of Square, the mobile payment tech company. The group, called The Square Registers, belted out its first number, “Take Me Home,” with soft harmonies and a single member [...]