Explain all default values in README.md
Make destroy default

The safest thing is to destroy all evidence, so destroy by default.
Fix temp dir location not joined correctly

The os.tmpDir() and 'id_rsa' where concatenated without a slash. Use the path.join method to join the directories which will add a slash if needed.
Merge pull request #1 from pjvds/master

Clarify README, destroy by default and fix temp dir bug