Added image caching with a sliding preference bar for choosing size

The start of resolving issue #29. This uses RAM cache to store images in RAM and allows the user to select how much RAM to use based on a slider preference. We'll still need to add DiskCache, but I wanted to see what we thought of the SliderPreference (pulled it from robobunny, who posted it for anyone to use) or see if we wanted to use something else.