Fix Sinon.JS issue 624

By first copying all the properties from `source` to `target` and only
then setting specific functions for `now`, `toSource` and `toString`,
we're not overwriting the custom implementation of those with the ones
from `source`.
Merge pull request #14 from mroderick/fix-sinonjs-624

Fix overwriting custom `Date.now`, related to Sinon.JS issue 624