Add Extra certificates for S/MIME so to include root CA, etc

[LeeNX]

Adding extra certificates so that S/MIME items can verify in Mail Clients without having to first add root CA, makes signed mail work better.

[Synchro]

Looks nice - any chance you could write some tests to go with it? If you're looking into S/MIME in general, can you do anything towards #72 and #127?

[LeeNX]

I should add, that this is for signing of mail items using S/MIME, not encryption. I will have to investigate how to write any tests and see if there is any info I can give/help with the above issues.

[Synchro]

I know signing isn't the same as encryption, but it will have much in common. I suspect the DKIM clash will be down to overlapping headers. Unfortunately we can't change existing function names as that breaks backward compatibility, but we can add new ones (PSR-2-style please!).

To write tests, add them to the phpmailerTest.php script in the test folder. You can run them locally with phpunit (set up a fake mail server as described in docs), or you can push to your own fork on github and travis-ci will run the test suite aginst your code, though that's quite a bit slower than running it locally.

[Synchro]

This was fixed in #376