Added workaround that detects if Matplotlib is flipping RGB images incorrectly
Remove origin='lower' since that is something specific to FITS files
Remove Header reading workaround
Update rgb.py

Replaced installed_pil with pil_installed so that APLpy is correctly aware of the PIL module.
Force origin='upper' for show_rgb to override any matplotlibrc settings
Merge pull request #101 from astrofrog/detect-rgb-flip

Workaround for matplotlib bug with flipped RGB image