DB merger and management

The db source file has been added to the standard dependency stack and
is no longer considered a plugin; the readme has been updated
accordingly.

This commit introduces the DBManager context, which acts as a unified
container for multiple DBSession objects and provides a means to manage
them.  The intent of this new [static] module is to allow for multiple
concurrent databases to be specified as sources for assets (see
DBManager documentation for more details).  Use of DBManager.find() will
search all active sessions, thus creating the equivalent of a query
against a single virtual database consisting of all files touched by the
active sessions.