Add a timestamp into ga.trending collector output

This adds a _timestamp into the ga.trending collector output as this has
been a required field since the json schema work has gone in.
This does not use the _timestamp in the _id field so it should not
affect the use.
Merge pull request #84 from alphagov/include-timestamp-in-ga-trending

Add a timestamp into ga.trending collector output
Empty data set before posting trending

If it is not removed, when a page path receives no traffic it will still
be included in the output with it's last values.
Merge pull request #85 from alphagov/empty-trending-data-set

Empty data set before posting trending