Amend `Flamsteed#_addMetaData` `t` assignment

* Use `Date.now()` instead of `window.performance.now()` 
  * `window.performance.now()` returns seconds from `navigationStart` - more complicated & unnecessary
* Amend `t` assignment check
  * Use `dot` syntax within `undefined` check
  * Check for empty value
* Remove logic from `Flamsteed#time` method
  * Represent as alias of `Flamsteed#log` within specs

References
* http://updates.html5rocks.com/2012/08/When-milliseconds-are-not-enough-performance-now
This line can just be `if (!data.t) {}` as both of those are falsy