I bet you want it to work for anyone playing your map, so what Feithless suggested might only work for you... Anyone downloading your map won't have these settings. :(
Anyway, I think what you want should work exactly the same way as using WASD for a unit, as Feithless also said. Create a camera on your map, and create triggers that will pan the camera to: actual position of the camera + small offset in the direction bound. Make some variables that will check if a key is still pressed (the more often you check, the more precise it is... unfortunately it is also more laggy, especially on Bnet), and repeat the camera movement process if so.
EDIT: I searched the link to the tutorial I used for WASD movement on my own custom map, but it seems to have been removed/updated with another method (obviously better, since the old one was laggy). Anyway, there are at least 3 ways to do a WASD movement, and each are explained on SC2Mapster... The method using Data editing might not work to move a camera, but you're free to try (who knows... it's an Actor like everything else, I guess!).