Turn ExtendedSetting public for plugins

[Hugo Ribeiro]

Hi,

I want to propose set ExtendedSetting public for plugins, to be possible get complex information like OAuth, etc.

Now is only in kupfer.core.settings (wiki say: don't import core) and isn't properly working.

I changed my to make this works:
Core changes
https://github.com/hugosenari/kupfer/commit/409fdb9e78355736d20082e892568998aabaa81b
Ugly changes, ugly because require plugin call one param as function (callback)
https://github.com/hugosenari/kupfer/commit/f9d931e7bfd9525a9552e0ee5ab563fb122d179e

my plugin:
https://github.com/hugosenari/Kupfer-Plugins/blob/master/ymeme/ymeme/ymeme.py

Thanks.