Class rewritten to get rid of deprecated APIs and make use of current Facebook API

These are the differences with the original class:

- the "init" method is needed no more since new APIs get application id from app_id string property (in strings.xml)

- added a "me" method to get info about the current logged user

- there is yet no support to ask for extended permissions as I don't understand how they are supposed to be passed on current api

- there is no "showdialog" method because I don't know what it's supposed to do and how to use it. If still useful this must be copied/rewritten as well

I've used the plugin directly, without the JS wrapper, so more testing is needed, and maybe some modifications, too.