Take a look at AirDisplay, which does exactly what you need:
Retina and HiDPI support are coming very soon as per this blog post, which suggests the App is already in the review process:
"So, stay tuned. We need to wait through another Apple review cycle before this goes live but we think it’s going to be worth the wait."
An Instant Second Monitor
Use your iPad, iPhone or Mac as a second (or third) monitor with no messy cables or wires.
Touch your desktop
Use your finger to draw directly on the screen. Try it with a Mac calculator app, painting tool, or music control surface!
Orient it anyway you want
Air Display works in both landscape and portrait configurations. Just rotate your screen, and it automatically reconfigures.
No wires needed
With Air Display you never need to mess with monitor cables.