Add flag to read assets from local files.
Add a date/time picker to graph UI.

- It's using https://github.com/mugifly/jquery-simple-datetimepicker
- If input is empty, it returns the most recent data
Rename localAssets to useLocalAssets.
Renamed date_s to dateString.

To be more explicit.
Save and restore end date in/from url.
Use hidden input for end.

Instead of appending to url manually.
Prevent unnecessary reload.
Increase/decrease end date by 1/2 range.
Add missing semicolons.
Merge pull request #91 from prometheus/feature/datetimepicker

Add a date/time picker to graph UI