database.URLTable adds remove(). engine.URLItem: Exposes URLTable.
hook.py: Supports "replace" as part of get_urls().
database.py: Normalizes the URL strings (chfoo/wpull#21).
database.py: Refactors URL insertion.
database.py: Fixes top_url not set correctly.
testing/goodapp.py: Adds infinite handler for manual testing.
database.py: Forces using SingletonThreadPool for SQLite.
database.py: Abstracts the SQLAlchemy logic.
processor,robotstxt.py: Ensures RobotsTxtPool is passed to sessions.
Merge branch 'issue/21-db_refactor' into develop
changelog: Adds note about db change. Updates latest to 0.15.
Merge branch 'develop'
Bumps version to 0.15.