Override WhereNode from django.db.models.sql.where to use SOQL NOT.

SQL negation: SELECT foo FROM bar WHERE Status = ‘Awesome’ NOT( Having_fun = false AND Out_of_cash = false)
SOQL negation: SELECT foo FROM bar WHERE Status = ‘Awesome’ AND (NOT( Having_fun = false) AND NOT(Out_of_cash = false))
Test that exclude query construction returns a queryset
Fix typ in docstring.
Add conditional for Django 1.5 or 1.6 to override their version of WhereNode.as_sql
Merge branch 'pr/26'

Conflicts:
	salesforce/backend/compiler.py