converted to chrome.extension.onMessage [Fixes #145]

[engstrom]

No description provided.

[adam-p]

Thanks for the submission. This is the second-ever pull request to Markdown Here!

But... I think there's a bit more to do before merging it:

Part of changing chrome.runtime.onMessage to chrome.extension.onMessage is also changing chrome.runtime.sendMessage in the same way. So there are a couple more lines that need to be updated.

Have you tested with Chromium <= 25? We're going to need to verify that the fix actually works.

[engstrom]

Changes are tested and working on Version 25.0.1323.1 (167142).

[adam-p]

I also tested on Version 24.0.1291.0 (160933) and the fix seems good.

I have merged this into the development branch. This will get merged into master when I prep the next release, which should be pretty soon. (This PR would get closed automatically at that time anyway, but I'm going to close it manually now.)

Thanks again.