Implement ks.date_range()

[xinrong-meng]

    >>> ks.date_range(start='1/1/2018', end='1/08/2018')
    DatetimeIndex(['2018-01-01', '2018-01-02', '2018-01-03', '2018-01-04',
                   '2018-01-05', '2018-01-06', '2018-01-07', '2018-01-08'],
                  dtype='datetime64[ns]', freq=None)

    Specify `start` and `periods`, the number of periods (days).

    >>> ks.date_range(start='1/1/2018', periods=8)
    DatetimeIndex(['2018-01-01', '2018-01-02', '2018-01-03', '2018-01-04',
                   '2018-01-05', '2018-01-06', '2018-01-07', '2018-01-08'],
                  dtype='datetime64[ns]', freq=None)

Please note that currently, we cannot handle freq properly, this is a known issue.

[ueshin]

Nice!

[ueshin]

LGTM, except for the style issue, also pending tests.

[ueshin]

Thanks! merging.