Added redmine integration

This is an attempt to address #26.

I'm not exactly sure how this should be merged in since there's no "main" central implementation of Redmine (that I'm aware of).  This gets at #47.  What I've written here is integration for redmine.org, which is the bug tracker for the project itself.  Therefore, this integration would only be useful for the maintainers of the project.

However, it's trivial to tweak the code for other individual implementations.  See https://github.com/WatchSteveDrum/toggl-button/compare/master...eecworld for an example (the manifest.json and background.json are the only files that actually matter).

Maybe this could go into a separate branch or something?  Maybe it warrants some explanation in the README?  If you do want to merge it, maybe you might want to wait until the permissions change again from another pull request since I doubt it's worth everyone having to re-enable their extension just so nobody at redmine.org can use it.