Put Preferences in subscreen

This is related to issue #331 
I used preference-headers as suggested in the issue and created two submenus. One is General and one is Advanced settings. A summary can be added under these options to describe what each is. I wasn't sure what to add in this case. 
Another option would be to just move the Advanced settings to a simple Preference and redirect through that to a separate PreferenceActivity but that's not so related to the new standards. 

I had to write some small pieces of code twice because there is the distinction between preferences running on android < 3.0 and > 3.0. It turned out to be a bit larger as a commit because of this need for backwards compatibility.

I tested it on android 2.3.2, 4.3 and 4.4.2 and on tablets and it seems to work as expected. 

Hope it's ok,
I can improve it if you find any problems.
Cheers,
Paul.