Add the Service::Update class

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Setup and teardown etcd & discoverd between tests

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Support registering attributes

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Support service attribute filtering

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #2 from lmars/attribute-support

Attribute support