Create all_dates function on datasets.
Use it both for json representation of datasets and to show in templates (via timerange context variable)

This fixes #684 on github
Created new resource: /<dataset>/timestamps.json that returns timestamps for a dataset
Reverted to older version to get min and max date, but moved it to the dataset model file (exposing it with timestamps() method)
Added created and last_modified timestamps to <dataset>.json
Removed timestamps resource (and all_dates function).
This is already possible to achieve by fetching /<dataset>/time.distinct
Wrap timerange in a try clause in case something bad happens
Merge pull request #685 from openspending/684-timestamps-in-json-dataset

Timestamps in json representation of dataset