I recently had a friend text me "ima go to [the local pub]. U in?" I understood it just fine as meaning "I'm going to", but being an old fogey, I'm not sure if this is just txtspk or if ima is something people actually say.

Is this just a texting abbreviation? If people really say this, how common is it and where is it mostly used? For instance, can I say it at the office, like "Ima get the spec to you by Thursday." Or is it more casual than that?