Changed the default name of primary key SF_PK='id'

I see in our README that this will be postponed to django-salesforce == 0.6. OK. Version 0.5 can be released now without this or with. It is not necessary to pull this now. Then it would have to wait until exactly before 0.6.

I'm going ahead. I wrote backward compatibility tests for SF_PK='Id' and this is for me the end of thinking about 'Id' until it fails. I developing everything only with lowercase. Maybe some feature come (e.g. fix of issue 55) that will not support "Id" easily.

I know that my change in README about primary keys is still obsoleted by your edits. Please fix simply that default is lowercase 'id' after this commit.