Only use strftime if in browser environment, for use in Node.js

[tomblanchard]

I am using liquid.js in a Node.js environment and am getting the document is not defined error when compiling templates due to strftime relying on the DOM, document doesn't exist in Node.js, my changes make is so strftime is used only when window is present (i.e. in a browser environment).

[mattmccray]

Does this still build correctly and pass all the tests?

[tomblanchard]

@mattmccray Sure does.