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

example/gramps/data.gramps

@@ -3,7 +3,7 @@
 "http://gramps-project.org/xml/1.7.1/grampsxml.dtd">
 <database xmlns="http://gramps-project.org/xml/1.7.1/">
   <header>
-    <created date="2015-08-01" version="5.0.0"/>
+    <created date="2016-02-06" version="5.0.0"/>
     <researcher>
       <resname>Alex Roitman</resname>
       <resaddr>1122 Boogie Boogie Ave</resaddr>
@@ -901,7 +901,7 @@
       <objref hlink="_HHNT6D73QPKC0KWK2Y"/>
       <childof hlink="_S7MT6D1JSGX9PZO27F"/>
     </person>
-    <person handle="_PSNT6D0DDHJOBCFJWX" change="1370206720" id="I0037">
+    <person handle="_PSNT6D0DDHJOBCFJWX" change="1370206720" id="I0037" priv="1">
       <gender>M</gender>
       <name type="Birth Name">
         <first>Edwin Michael</first>