Integrate daylight saving support

Integrate datainsight_recorder v0.4.1 which brings in support for
daylight savings time. DateTimes are stored in the database as
Europe/London timezone.
Add more info to error logging on save failure

DataMapper doesn't tell you the root cause, just that the save failed.
Be explicit about datetime offsets in tests

- Change tests to use explicit datetimes with relevant offsets
- Localise datetimes when creating measurements for integration tests
Merge pull request #7 from alphagov/dst-upgrade

Integrate daylight saving support