Merge pull request #21 from DarwinAwardWinner/lexbind-fix

Complete the docstring for "async--get-function"
Revert "* helm-async.el: Fix error handling."

This reverts commit 242ae734273162f4ec027618610a099c9a73557b.
Revert master back to 242ae73
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.