Change entry point of API for stateless design

Since the entry point (org.hidetake.groovy.ssh.Ssh) has static instance of service, it may cause state problem. So it is better to create an new instance for each usage not reuse an instance.

This pull request replaces static properties with instance creation methods as follows:

* `org.hidetake.groovy.ssh.Ssh.ssh` -> `org.hidetake.groovy.ssh.Ssh.newService()`
* `org.hidetake.groovy.ssh.Ssh.shell` -> `org.hidetake.groovy.ssh.Ssh.newShell()`