Add jar-manifest-mode

[omajid]

Add jar-manifest-mode that implements a major mode for editing Manifest.mf files used in JARs.

[purcell]

Thanks. Quick feedback:

• For setq-local you need Emacs 24.3 or greater. Rather than declaring a dependency on that Emacs version, I suggest you use the classic (set (make-local-variable 'name) value) method instead
• prog-mode is only available in Emacs 24, so you should add a ;; Package-Requires: ((emacs "24")), or use a workaround like in haml-mode.

Looks good otherwise. :-)

[omajid]

Thanks for the feedback! Any suggestion on how to identify these compatibility issues beforehand?

Fixed with this commit.

[purcell]

Any suggestion on how to identify these compatibility issues beforehand?

I hope to add some corresponding checks to flycheck-package when time allows.