FEATURE: add iframes support on Firefox

This commit introduces a complete refactoring/cleanup of the Firefox version, it supports Ember apps embedded into iframes (Ember libraries detection and Devtool panel connection) and it has an optional tomster icon in the locationbar.

The tomster icon can be enabled/disabled from the addon preferences, and it will be visible when an Ember app is detected in the tab or in an iframe inside the tab.

Additional Notes:
- move shared resources (images and the *in-page-script.js*) into a new *dist_common* directory 
- cache downloaded Mozilla Addon SDK into *.mozilla-addon-sdk* between builds
- workaround bug on *select* unstyling issues on Firefox (https://bugzilla.mozilla.org/show_bug.cgi?id=649849), this needs to be tested on OSX and Windows to be sure the current workaround is working correctly on all the supported platforms  

Requirements:
- Firefox >= 30.0 (needed for the "evalInWindow" method used in the *content-script*)

Tested on:
- http://jsfiddle.net/9zHj9/1/

to be merged into #182