Added beforeDrag callback modeled after accept callback.

This callback gives you the ability to conditionally disable dragging of specific nodes. This is similar to how the accept callback allows conditionally disabling drop.
Added minified files as well.
Merge pull request #156 from RallySoftware/master

Added beforeDrag callback modeled after accept callback.
Fixed #167
fix the way attributes are watched #164
Merge pull request #168 from jrproctor/tree-options

Improve the way attributes are watched
Fixed #150
Merge pull request #170 from adivspan/master

Fixed #150
Include sass files to be able to @import using sass
Add removed event handler
Merge pull request #172 from gus4no/master

Include sass files to be able to @import using sass
Merge pull request #174 from wcandillon/master

Add removed event handler
Add Dragging Elements to configurable Element instead of 'body'
Merge remote-tracking branch 'jim/master'

Conflicts:
	demo/dist/angular-ui-tree.js
	demo/dist/angular-ui-tree.min.js
	dist/angular-ui-tree.js
	dist/angular-ui-tree.min.js
	source/directives/uiTreeNode.js