Ignore standalone `@` in coffeescript files

Since `@` in Coffee can compile to `this` or `_this` in JS, it actually cleaner to use it, as it reminds everyone that they are dealing with a CoffeeScript construct.

An explicit `this` looks as if it's going to look the same in JS, but in reality it is compiled exactly the same as `@` (so it can end up as `_this` if using fat arrows, thus potentially confusing the user).