New Upload plugin base classes

Upload plugins (i.e. tool connectors) can now rely on a number of
framework classes to do some of the work for them:

* The ContentService can be used to create nodes, notes, issues and
evidence so the plugin writer doesn't have to deal with the base clases
directly.

* The TemplateService was a Pro-only feature until now, and it lets the
user map between the output produced by the original tool and whatever
input they want for their Dradis project.