Fixed #14, improved #2,

Important fixes are:  
#14 repeated delete doesn'raise anything (improved a small part of #42)
#2 dumpdata works easier

I haven't committed the complete multi db support yet, but only parts related to #2

(The old branch "generic-foreign-key-support" is completely merged and can be safe deleted from [merged branches](https://github.com/freelancersunion/django-salesforce/branches?merged=1)