date_range should (at least optionally) deal with right-open intervals
It is confusing that, albeit the name suggests otherwise, `date_range` behaves differently compared to the standard Python `range` generator in that it returns the right-closed interval.

@filmor you can submit a PR if you would like, but this is essentially label based indexing, so the python convention doesn't apply here. That side, having a `closed=left|right'` would not be a bad option
 I still have to build some test-cases for the new inputs before this is ready for merge.
 I think `closed` can only apply to the right side (too confusing othewise). Also pls add a release notes and v0.13 entry (enhancements)
 The current commit contains a release note, but I don't understand your comment on the parameter. What is wrong with being able to have open, left-open, right-open and closed intervals? There are uses for all of them and the interface seems pretty obvious to me …
 take a look at `tseries/resample.py/TimeGroup`

`closed=None` is the default, can be 'right' or 'left'....let's go with this so the API is not changed

`None` is your equivalent of `True`, `False` (meaning both open) is not supported

can't change the API ATM different from what exists in the rest of pandas.
(you are welcome to do that in 0.14, but that would require some more discussion)
 @filmor need some tests for this; e.g. with closed taking on each value and that the generated range is correct (for several freqs should be good).  should go in `tseries/tests/test_daterange.py`
 Could you have a look at the Travis output? An error occurs in a place, where I didn't change anything(tm), for some reason the `Timestamp`s in `DatetimeIndex._generate` are converted back to `datetime.datetime` without a `tz` attribute, leading to an error. However this only seems to happen after the `start += offset` or `end -= offset` call ...
 @filmor did you fix the bug?
 @filmor how's this coming?
 thanks for the PR!
