Inject bytecomp variables (#44).

* async-bytecomp.el (async-byte-recompile-directory): Do it.
Use always the args 0 and force for byte-recompile-directory.

* async-bytecomp.el (async-byte-recompile-directory): Remove confusing
args used in byte-recompile-directory and add docstring.
(package--compile): Remove old args of async-byte-recompile-directory.
Merge branch 'master' of github.com:jwiegley/emacs-async