Don't rely on async.el being in load-path.

This uses "symbol-file" instead of "find-library-name" to figure out
where async.el is. This works even if async.el was loaded without adding
its directory to the load-path.

Fixes #15.
Merge pull request #23 from DarwinAwardWinner/no-load-path-fix

Don't rely on async.el being in load-path.