Read only fields etc. for issue #11, instead of pull request 13

These are commits that I definitely consider useful and mature for master, after the current big re-edition.

All commits are **atomic** so that it is easier to describe it and understand it. Every known typo has been finally fixed in the same commit where it arose. Tests for new features or for fixes of very old code are usually in the same commit with that fix. If only `tests` and `testrunner` directories will be checkouted, the tests fail. If the complete commit is checkouted, the tests pass. All my unuseful or unused code has been moved to development branches that start after this pull request. Any undesirable feature can be picked by 

I rebased your (sbussetti's) commits that depends on Phill's pull request https://github.com/freelancersunion/django-salesforce/pull/13 based on my first sample to sample hynekcer/django-salesforce@ad1d4ac0e3e00b0e828986a8725a71f6efcb262e
Some your code is not rebased because I removed the explicit custom code TimbaSurveys and the name is completely configurable by local_settings. I also removed settings_secret because I missed your local_settings first and that your implementation is sufficient for passwords etc.
 I would like to ignore my old code history although I do not remember any its serious bug except history readability.

I verified all commits by tests. Starting after adding *tox* test configuration, all commits are tested on five different virtual environment.