Redmine

I'm just starting out with developing plugins, so please excuse me if this has already been asked&answered: I could not find too much on the topic in the docs:

I'd like to develop the plugin outside of a redmine installation, is that possible? Right now, the code for the plugin must live inside a redmine-installation in plugins. Only from here it can be tested, as it needs the redmine-infrastructure. This especially means, that the whole redmine-installation needs to be checked in, along with the plugin, to be able to setup things like continuous integration. Am I getting this right or is there any other way to set this up?

Ideally, I'd like to have the plugin, as a standalone gem with tests running separately and the redmine core-classes added as a gem-dependency. I've seen some plugins achieving something similar using heavy scripting-fu to pull in redmine (eg. https://github.com/backlogs/redmine_backlogs) - is that the way to go?!

And in the end, instead of manually copying the plugin into the plugins-directory, I'd like to develop the plugin as a mountable engine and simple add it to redmines gemfile (or preferably even to a separate plugins-gemfile)- is that already possible with redmine 2.x?

Any pointers on how to achieve that would be highly appreciated!!

cheers
Stefan