Google+ Share button

Hi,

I have added a simple Google+ Share button into each item to simplify the share action of an article :

![Share button](https://f.cloud.github.com/assets/988914/268433/6194eba8-8f2f-11e2-83ce-d95895ca502e.png)

I have also inverted j/k and t/shift+t. I know (and understand) your position about the shortcut and the next item behavior.

If you don't want the j/k and next item placement modification just ignore modification in the "public/js/selfoss-shortcuts.js")