Add frequency attribute to examples.
Bulk write method wasn't a brilliant idea. Removed!

The write method now handles the case where a new file needs to be
created.
Use entry format str var instead of hard coded str.
Remove print statement from unit tests.
Fix bug with appending non-daily data.
Add list_ids method.
Use a UUID to represent a timeseries instance.

This is with the intention of being able to associate multiple
attributes with a unique timeseries instance.