Implement Singleton pattern inside synnefo.lib.singleton.

Whenever a class inherits from ArgBasedSingleton,
attempts to call the class do not always create a new
class instance, but return pre-existing, already made objects.

All of the arguments to the class call must match, to return a
pre-existing object. Rationale: when the ModularBackend from
snf-pithos-backend is converted to inherit from ArgBasedSingleton,
attempts to create a new backend instance with the same settings will
re-use the existing backend object.

Signed-off-by: Chris Stavrakakis <cstavr@grnet.gr>
Signed-off-by: Kostas Papadimitriou <kpap@grnet.gr>
Signed-off-by: Vangelis Koukis <vkoukis@grnet.gr>