Supported more Salesforce databases in an application.

- The setting SALESFORCE_DB_ALIAS is the name of the primary Salesforce
  database. It is possible to completely change the precedence of databases
  and 'salesforce' can be the name of secondary SF database.
- The SF database can be selected by `.objects.using('some_sf').filter()`
  and by `.save(using='...')` for saving new objects.
- Foreign keys correctly relate to objects in the same database.
- An obsoleted auth session causes reauthentification of the right database.
- Some some features can be tested only if more Salesforce databases are
  defined in DATABASES settings.
- Modified auth.authenticate parameters so that the old usage should raise
  a clear exception. (though a compatible solution was functional, but it was
  less readable.)
Simplified query.py
Support for allow_migrate in Django 1.7 to ignore some commands with SF.
Fixed inspectdb default ForeignKey to "on_delete=DO_NOTHING"
Moved print output to stderr, due to inspectbd
Fixed comments and whitespace and typography.
Removed an old function unused for two years.
Merge pull request #58 from hynekcer/multiple-databases

Multiple Salesforce databases support