Do not store a direct reference to Date.now

The adition of `_.now` made it much harder to use fake timers when testing (e.g., http://sinonjs.org/docs/#clock), because we need to stub `Date.now` before loading Underscore. This is not practical at all and doesn't allow for using fake timers only in some tests.

This patch adds a wrapper function around `Date.now`.

I understand that this may incur a small performance penalty, but I think that being able to test code that uses `_.now` (directly or indirectly) is more important.