Add human relative time formatting utility

[jacobtomlinson]

Added human relative time formatting utility dask.utils.format_time_ago.

>>> from dask.utils import format_time_ago
>>> from datetime import datetime, timedelta

>>> now = datetime.now()
>>> format_time_ago(now)
'Just now'

>>> past = datetime.now() - timedelta(minutes=1)
>>> format_time_ago(past)
'1 minute ago'

>>> past = datetime.now() - timedelta(minutes=2)
>>> format_time_ago(past)
'2 minutes ago'

>>> past = datetime.now() - timedelta(hours=1)
>>> format_time_ago(past)
'1 hour ago'

>>> past = datetime.now() - timedelta(hours=6)
>>> format_time_ago(past)
'6 hours ago'

>>> past = datetime.now() - timedelta(days=1)
>>> format_time_ago(past)
'1 day ago'

>>> past = datetime.now() - timedelta(days=5)
>>> format_time_ago(past)
'5 days ago'

>>> past = datetime.now() - timedelta(days=8)
>>> format_time_ago(past)
'1 week ago'

>>> past = datetime.now() - timedelta(days=16)
>>> format_time_ago(past)
'2 weeks ago'

>>> past = datetime.now() - timedelta(days=190)
>>> format_time_ago(past)
'6 months ago'

>>> past = datetime.now() - timedelta(days=800)
>>> format_time_ago(past)
'2 years ago'

[mrocklin]

Minor docstring comments!

In general this seems like a nice thing to have. Thanks for centralizing it here rather than elsewhere.

[mrocklin]

+1

[jacobtomlinson]

Thanks for the review @mrocklin. Updated!