testing/server: simplify the failures logic
testing: fix some golint warnings
client: improve error handling on version parsing
container: update State to match later API changes
container: add PauseContainer
container: add test and improve State.String
container: add UnpauseContainer
testing/server: add route for pausing a container
testing/server: add route for unpausing containers
testing: add method to notify containers through a channel

Related to issue #105.