Add template capability.

When registering blueprints with register_blueprint, it seems that the
template folder passed as argument, does not work as expected (even
when passing an absolute path). blueprint_helper has been created
to help with this problem: Its set_template_folder will set the
template folder on an instantiated blueprint according to:
1) the app's location,
2) the app's TEMPLATE_FOLDER config entry,
3) the blueprint's name.

So for instance, a blueprint named "foo" has this template folder:

/<app location>/templates/foo.