Added AutoHotkey highlighting, resolved #287

The first commit adds AutoHotkey syntax highlighting to Prism. The file prism-autohotkey.js is big because of the large amount of keywords in ahk. Please merge it with the mainstream.

The second commit fixes the issue #287. I have chosen a **distinguishable** light red color for the function token. You should like it. 
Here is a screenshot showing ahk highlighting (red color are the function calls)
![ahk](https://cloud.githubusercontent.com/assets/4047597/3522639/874493fa-074f-11e4-9f90-2b80f21040a5.png)