Added a close method to DockerClient

Give programs using DockerClient an opportunity to release resources claimed
by the http client created inside DockerClient.
Merge branch 'docker-client-closeable' of
https://github.com/kimble/docker-java into
kimble-docker-client-closeable

Conflicts:
	src/main/java/com/github/dockerjava/client/DockerClient.java
t push origin master:Merge branch 'kimble-docker-client-closeable'