Support for Django 1.7 and additions for Python 3.3+

The previous pull request #34 is correct in my opinion. This pull request continues on it. It also depends on the urgent #35

Two improvements for transport (option for disable IPv6 and use retries for network problems) are not so important as I thought, because the problems with IPv6 were only temporary last week, when their DNS answered by one IPv6 and more IPv4 adresses for test.salesforce com questions. Currently it is disabled again by SF. Probably they also found it was problematic. (Google's implementation of IPv6 worked.)