Don't add where clause if subject is null. Fixes #90.
Removed unneeded null check.
Test for query with null subject

refs #90
Simplifying test
Merge branch 'issue/90-null-subject-query-test' into issue/90-null-subject-query
Test harness for PersistentStore.QueryBuilder

Makes QueryBuilder SQL directly testable.

Refactored PersistentStoreTest to an abstract base class PersistentStoreBaseTest so the query builder tests can reuse the same database setup.
Close cursors in test
Allows indexes to have null values

Adds null value indexer to test Note model.
Allow searching for null indexes

If a query is using an EQUAL_TO, NOT_EQUAL_TO, LIKE or NOT_LIKE comparison to a null value then the SQL will look for matching objects that have a null value in an index at that value
Test shouldn't delete the database
Merge pull request #91 from Simperium/issue/90-null-subject-query

Supporting null indexes and query values.