Code cleanup and de-"clever"ification for mustache_helper.js

* fixed indentation
* consistent spacing "if (", ") {", "function ("
* explicit return statements, esp. for "!f()" at the end of a function
* a && b -> if (a) b

Also simplified and separated the "firstexist" and "firstnonempty" helpers from a can.each loop, so IDEs could pick up their definitions.

Commented several helpers.

Only style, no functional changes.
I just saw your comment here https://gist.github.com/danring/631a43980245e680f6ec#comment-1337151 

Personally I don't care that much, but in google's style guide it's always `function() {`.