Initial pull request - created plugin base

Hello gentlemen, 

Main goal of this piece of code was to make a working proof-of-concept of a Firefox plugin, which includes: 

 * automatable and repeatable build 
 * facilities for dependency management, some configuration management and static checks. 
 * basic plugin interface of a sidebar and linked buttons
 * plugin code structure - what goes injected, what's working on sidebar, etc
 * first version of a detector (not too optimal yet), capable of catching different types  of images (`<img>` elements, all elements having css `background: url('..')` including `data:` schema
 * stubs for querying. 
 * integration to a firefox workflow (tabs, windows, navigation, etc.) 

From this pull req and on, I think we'll be making smaller ones, per-feature rather than general requests. 

Encourage everyone to make a code review of this and make any code suggestions! 

Thanks, 
Alex.