Ofgem certificate date parsing

One of the ofgem certificates with year 2011 and month 9 had a date with a time other than 00:00:00 which broke the parser so I just removed the time part entirely:

```
Traceback (most recent call last):
  (trace omitted)
  File "/usr/local/lib/python2.7/dist-packages/pywind/ofgem/CertificateSearch.py", line 96, in get_data
    self.certificates.append(Certificates(detail))
  File "/usr/local/lib/python2.7/dist-packages/pywind/ofgem/Certificates.py", line 58, in __init__
    self.issue_dt = datetime.strptime(self.issue_dt, '%Y-%m-%dT00:00:00')
  File "/usr/lib/python2.7/_strptime.py", line 325, in _strptime
    (data_string, format))
ValueError: time data '2011-12-06T16:44:00' does not match format '%Y-%m-%dT00:00:00'
```