Implemented defer(..) and only(..) queryset methods.

Output fields can be specified by also in some internal methods.
Enabled and improved tests for deferred fields queries.
Tested the minimal number of queries for defer(..) and only(..)
Added settings SF_PK for configurable primary key name: 'Id' or 'id'.

The similar undocumented feature for individually configurable names sf_pk
by models has been removed.
Implemented multiple inheritance of database Model from more abstract models.
Fixed SalesforceModel for proxy models.
Added tests for multiple inheritance (abstract models, proxy, mixins).
Fixed unimportant inconsistent spaces/tabs in Python code etc.