Easier to set temp dir, and all temp files are unlinked after use

Hi Andris, thanks for 'pem'. I noticed that the existing fs.unlink at the end of each openssl call wasn't actually doing anything. This PR makes it so all the temp files actually get removed. 

I also added an environment variable, PEMJS_TMPDIR, that lets you set a separate temp directory for pem from the global node.js one.

Cheers,
Anand