Add symlink support (Fixes #42)

It's missing the option for relative symlinking, but this should hopefully do the trick for now. Merry Xmas!
Other support functions in lib/dest don't have unit tests. I thought since the code coverage didn't change (and no new functionality was added) extra tests wouldn't be needed. The existing feature tests in `test/dest.js` and `test/symlink.js` seem to cover it just fine too.

If you still think it's required I can whip something up tomorrow.