Added context use_ipopo(bundle_context)

Intends to replace the get_ipopo_svc_ref method, using the way simpler
'with' keyword.
It also insures that unget_service(ipopo_ref) will be called in any case,
while exiting the block.

Example:
with pelix.ipopo.constants.use_ipopo(bundle_context) as ipopo:
   ipopo.instantiate(...)
ServiceReference usage per bundle is now counted

Instead of keeping a "using"/"not using" list, the bundles are now associated
to a counter.
A ServiceReference is considered not used by a bundle if there has been as
many unget_service() as get_service() calls.