timestring: work around 29th February bug

[GraemeWatt]

The timestring.Date function is currently called in four places:

hepdata/hepdata/modules/inspire_api/marcxml_parser.py

Line 67 in 896e7f6

return timestring.Date(datafield.find_all(code=code)[0].string).year

hepdata/hepdata/modules/inspire_api/marcxml_parser.py

Line 180 in 896e7f6

return date, timestring.Date(date).year

hepdata/hepdata/modules/converter/views.py

Line 281 in 896e7f6

year = timestring.Date(record['creation_date']).year

hepdata/hepdata/modules/converter/views.py

Line 514 in 896e7f6

year = timestring.Date(record['creation_date']).year

There is a bug when timestring.Date is called on 29th February and the year passed is not a leap year:

>>> import timestring
>>> timestring.Date("2001-07-01")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/graemewatt/.virtualenvs/hepdata/lib/python2.7/site-packages/timestring/Date.py", line 150, in __init__
    new_date = new_date.replace(year=year)
ValueError: day is out of range for month

This bug should be worked around or the dependence on timestring.Date removed.

See also codecov/timestring#38, codecov/timestring#39, codecov/timestring#62, codecov/timestring#63.