Changed cert_db to use Certificates and log_ids.
Keep entries in DISABLED_TESTS sorted.

Also removed a trailing backslash.
GMock does not pull in GTest.
Merge branch 'master' into super_duper
Small cleaning up of blocking_callback.h and fake_etcd_test.cc.
Add a Notification class.

This is similar to util::BlockingCallback, but simpler.
Replace BlockingCallback with Notification in fake_etcd_test.

And also remove BlockingCallback. It is an artefact of an older callback
system at Google, Notification is easier.
Add a util::Task class to manage async operations.