Django 1.8 compatibility.

[krischer]

django.db.backend is no longer available on Django 1.8.

This PR instead uses the vendor attribute of the database connection. I hope this works everywhere but if it does it would also warrant a new pypi release.

[barsch]

unfortunately not working for me on Django 1.8 even with this patch

>>> query
[<ContinuousTrace: TA.A25A..VCO | DateTimeTZRange(datetime.datetime(2010, 3, 24, 23, 59, 38), ...]
>>> query[0].channel
'VCO'
>>> query.filter(channel__like='VC%')
[]
>>> query.filter(channel__like='VC_')
[]
>>> query.filter(channel__like='VCO')
[<ContinuousTrace: TA.A25A..VCO | DateTimeTZRange(datetime.datetime(2010, 3, 24, 23, 59, 38), datetime.datetime(2010, 3, 25, 0, 0, 58), '[)') | 0.1 Hz, 9 samples>]
>>> query.filter(channel='VCO')
[<ContinuousTrace: TA.A25A..VCO | DateTimeTZRange(datetime.datetime(2010, 3, 24, 23, 59, 38), datetime.datetime(2010, 3, 25, 0, 0, 58), '[)') | 0.1 Hz, 9 samples>]