Problem with brew belts as suggested above is that they don't really allow you to control your fermentation temperature. Sure, they'll warm it up but as there is no feedback loop from the brew temp to the heater they can cause the temp to rise too high.
As for saving time... they're not really designed to save time though it is true that your brew will ferment more quickly the warmer it is (to a point). However, there are unwanted by-products associated with too warm a fermentation and correct temp combined with patience will win every time.
You will have read that some use an aquarium heater submerged in a tub of water. This is better than a brew belt as you will have feedback to the heater which incorporates a thermostat thus giving you temperature control.
Keep your brew between 18-21 and let it do its thing for couple of weeks.