Add comments.
Add a variable for storing passphrase in a node.
Add Empty key class to represent SSH key file in code.
Merge branch 'master' into diff-keys

Conflicts:
	src/Altax/Module/Server/Resource/Node.php
Fix conflict
Fix conflict
Support different keys for nodes
Add tests
Add tests
Update dependence packages version. Create phar for verion v3.0.11
Fix const value for git commit hash . I broke it previous commit.
Tiny update
Rebuild phar file
Merge pull request #32 from kohkimakimoto/diff-keys

[WIP] Support different keys