Hide db._select(); Refactor db._tables; QuerySet; tests and bug fixes

db.select() has become db._select()

This commit refactors database._tables so that they can work properly
after a proxy or filter has been removed.

It adds abstract API called QuerySet which allows a variety of
selections and ordering of data.

Logging was added to sqlite to better see db access.

Additional fixes:

* clearing GenericDB._directory
* test_util_test off-by-one error

.travis.yml

@@ -31,7 +31,7 @@ script:
     -  mkdir -p ~/.gramps/grampsdb/
     # --exclude=TestUser because of older version of mock
     #                    without configure_mock
-    -  nosetests3 --with-coverage --cover-package=gramps --exclude=TestcaseGenerator --exclude=vcard --exclude=merge_ref_test  --exclude=user_test gramps
+    -  nosetests3 --nologcapture --with-coverage --cover-package=gramps --exclude=TestcaseGenerator --exclude=vcard --exclude=merge_ref_test  --exclude=user_test gramps
 
 after_success:
     - codecov