Allow underscore.js to be loaded

The wrapper around every script file supplied by the LiveScript Brunch
skeleton declares `exports` as an empty object in order to supress
errors in the LS prelude.

By declaring it as undefined, we can ensure that underscore.js does not
assign itself to `exports` instead of the window object, without
altering underscore.js.
Merge pull request #2 from jimmed/issue-1

Allow underscore.js to be loaded