Added a BundleActivator decorator in pelix.constants

The decorator instantiates the activator class and stores it as a
module-level member.
The name of this member has been renamed from "activator" to
"__pelix_bundle_activator__" (to avoid name collisions).
A warning is logged if a bundle uses the "activator" member name.
Updated the BundleActivators in Pelix bundles
Updated samples and tests bundle activators