Make compatible with Linux

Hey!

Thought you might want to take a look at this. I know I'm changing the file extension - it's the two values I've been using for years with Notepad++ and so this helps me use my already existing files automatically.

The other two commits are to solve the following two problems:
1. Providing the cache file causes ST2 to behave erratically as ST2 will load the cache file at the expense of parsing the .tmLanguage file.
2. Linux file extensions are case sensitive, so ST2 on my Arch install would not show the syntax in the Menu or in the "Set Syntax" command.

All this has led me to believe that ST2 parses the cache file for the actually highlighting, but the PLIST file for generating the menu.