Unify codebase with Safari and chrome versions

[owise1]

chrome - https://github.com/coreytegeler/add-art-chrome
safari - https://github.com/owise1/add-art-safari

since there are so many similarities it would be ideal to have all the code in one place. however, there are non-trivial differences related to what the content scripts and popup can access from the background process.

Mozilla's move to webExtensions looks promising for joining the FF and chrome versions. Right now though, FF isn't implementing all the necessary APIs.

[slambert]

Whenever you want to do that, you’re welcome to.