Add a start_date/end_date fields

Also added is a paragraph on dates

Fixes #29

We don't want to specify `"format": "date-time"` because we want to allow
partial date-times. Also, `make check` doesn't check the format anyways.
Add start_date/end_date to BritishColumbiaMosaic as an example
Round lat/lon to 10cm for XML conversion

This should help keep output consistent across machines
Reprocess xml and geojson with rounding
Rerun make on imagery.json

This was missed on 04115c30678e9fe7a305b4894263098857841cb8
Merge pull request #32 from osmlab/rounding

Rounding
Merge pull request #31 from osmlab/date_range

Date range
Start/end date is not necessarily imagery specific
Add a pattern for start/end date