Bug 732294 - Panel constructor lacks a way to set position of panel other than "centered"

Implemented the API described in [JEP Panel Positioning](https://github.com/mozilla/addon-sdk/wiki/JEP-Panel-positioning) and discussed in etherpad.
Merge pull request #826 from ZER0/panel-align/732294

fix Bug 732294 - Panel constructor lacks a way to set position of panel other than "centered" r=@gozala