Add manifest.json file and context menu

- Added initial manifest.json prototype with
  basic configuration parameters.
- Added init.js script responsible to create the
  context menu item when user right-click on a
  page.

Current commit will produce the following result (wish GitHub could handle image :
![screen-shot-2014-10-01-at-20 23 46](https://cloud.githubusercontent.com/assets/6195629/4482088/102f182e-49a5-11e4-84be-1dcee8b1a8ef.png)