Interval is now always seconds

Now, the interval in both iterators and in the ts object is in seconds.
This makes life easier as we don't need to * or / by 60. This may or may
not break pipelines (it shouldn't, as the iterators took seconds anyway).

Fixes #47

 Changes to be committed:
	modified:   tests/helpers.py
	modified:   timestream/__init__.py
	modified:   timestream/parse/__init__.py