Fix horizontal sorting. Closes #72.

So I changed the logic just a bit, updated all the corresponding script files, checked horizontal sorting with several levels of nesting and it works great! You can see for yourself [here, in updated demo page](http://zeird.github.io/angular-ui-tree/tree-horizontal.html). (I'll make another pull-request for gh-pages update).
My changes don't break any existing angular-ui-trees, because it doesn't do anything until you set ```data-horizontal=""``` attribute to target ```ui-tree-nodes``` (and add ```display: inline: block;``` and ```vertical-align: top;``` to corresponding ```css```-file). Don't forget to mention that in ```README.md``` if you pull my changes.
The only thing I can't figure out is how to move a ```tree-node``` to an empty ```tree-nodes``` properly (a bug probably, [look at this explanation .gif](http://i.imgur.com/Rlq3tta.gif)).