Solve classname clash, fixes #3138

When running CLI tasks under specific circunstances, Propel::autoload()
is registered before sfAutoload::autoload() in the queue of autoload
functions. Propel::autoload() includes its own BaseObject.php class that
happens to have the same name that our own lib/model/om/BaseObject.php
class.

In other words: QubitObject extends BaseObject. When it's invoked from
our tasks, Propel was loading:
sfPropelPlugin/lib/vendor/propel/om/BaseObject.php instead of
lib/model/om/BaseObject.php. I've renamed the Propel BaseObject class
to BasePropelObject. We are still using Propel 1.4 so I don't really
expect any update from the vendor.

See our wiki page /Autoload for more details.