add linux support (yes it's just renaming a folder)

HUD seems to work completely fine for me after doing this, including fonts.
Since Linux filesystems are case-sensitive, TF2 seems to just downcase everything and load from there.

The only (minor) problem is the icons on the main menu are slightly misaligned (see http://i.imgur.com/LIRgMhw.png ).
Not sure about Mac, but yaHUD said it had problems with Mac/Linux and that worked fine for me too, but not Mac users (according to the GitHub issues).