Merge pull request #9 from marckysharky/master

Page Impression Instance Amendments
Merge pull request #10 from marckysharky/master

Amend flamsteed serialisation to ensure event uniqueness.
Amend `Flamsteed#_addMetaData` `t` assignment
* Use `Date.now()` instead of `window.performance.now()` due to `window.performance.now()` returns seconds from `navigationStart`