Add tests for Realtime class
Add tests for summer and winter timezone handling
Implement fix for timezone and address comments

As for as we can tell, the process for generating realtime timestamps
used to go:

- generate timestamp now for utc e.g. 11:20 am +01:00 when BST would be 10:20 am +00:00
- change timezone to gmt without affecting the rest of the time e.g. 10:20 am +01:00
- send to backdrop
- backdrop converts to UTC but gets confused by the offset and thinks UTC is 09:20 +00:00
- save, timestamps as they are now will be sent to spotlight.

This puts the time coming to spotlight one hour back vs local time

The fix:

- generate timestamp now for utc e.g. 11:20 am +01:00 when BST would be
  10:20 am +00:00
- send to backdrop
- backdrop converts to UTC but this makes no difference
- save, timestamps as they are now will be sent to spotlight.

Tests force the system time to a summer time and winter one and check we
are sending the correct timestamp to backdrop.
Merge pull request #24 from alphagov/fix-timezone-issues

Fix timezone issues