Added (configurable) close on lost focus support

["Grigory Javadyan" <grigoryj ovi com>]

Hello,
I was searching for gnome-do alternatives and stumbled upon kupfer today. I really liked it except for one thing: the main window wouldn't close upon losing focus. That was a showstopper for me. I looked at the project's launchpad page and saw that some people actually requested this feature. I liked Kupfer so much that I decided to implement it. So I went to github, forked the project and did it!
The changes are basically the following:
* Added a new configuration parameter, "closeonunfocus" in the settings module
* Altered browser.py to (dis)connect the focus-out-event signal to/from _close_window upon startup/ settings modification
* Changed the preferences.ui file so that you can choose whether to enable "close on unfocus" from the preferences dialog. 
My kupfer's fork is located here: http://github.com/10098/kupfer
You can treat this e-mail as a pull request  :)

Many thanks,
Grigory 

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