allow to unset a previously set RANGE value

... and add the unset_range_test.py unit test exercising it

Reported by: Zdeněk Pavlas
Bug: https://bugzilla.redhat.com/928370
allow to use setopt(..., None) as unsetopt()

... and extend the unset_range_test.py unit test to exercise it

Reported by: Zdeněk Pavlas
Bug: https://bugzilla.redhat.com/928370
Merge pull request #29 from kdudka/bz928370

[RFE] support setopt(pycurl.RANGE, None)
Merge pull request #30 from p-push/global-ack-eintr

Global ack eintr
Merge pull request #31 from p-push/python-3-easy

Python 3 support
Python 3 support (src).

http://sourceforge.net/tracker/?func=detail&aid=2917775&group_id=28236&atid=392779

http://sourceforge.net/tracker/?func=detail&aid=3188495&group_id=28236&atid=392779