Add more tests

[mojavelinux]

• add test fixtures
• add tests for all operations (exec, shell, sftp, dest)
• add chai-fs for testing for the existence of files and directories
• add fs-extra for test cleanup
• rename gulp test task to samples

[mojavelinux]

Uh oh. sftp subsystem isn't working in Travis CI. Investigating...

[mojavelinux]

Travis is happy now!

[mojavelinux]

Should we test against Node 4?

[zensh]

Thank you. Node 4 is unnecessary.

[mojavelinux]

:tada:

[mojavelinux]

Now that I have the test suite set up, I have a list of tests I still want to add. I'll keep 'em coming.