It's the moment you've been waiting for. Google just announced the Nexus devices are getting OTA updates to 5.0, and now the full system images have been posted on the Google Developers site. With these files, you can flash your device back to stock even after you've done unspeakable things to it.
The images we have so far are for the Nexus 5, 7 (both WiFi models), 9, and 10. There's no Nexus 6 image yet. Devices like the Nexus 4 that aren't getting an OTA just yet are also still waiting on the system image. Binaries are also up for all the devices with factory images, save for the Nexus 9, which has the drivers on a separate system partition already.
After you get the appropriate image downloaded, you can flash it from your computer in just a few clicks. You don't necessarily have to wipe your device, but you might want to do so just to be safe. We've got full instructions right here if you need them.