Change null to undefined
Add new callbacks for collapse and expand.
It receive two parameters, the node collapsed/expanded and a boolean
indicating if all nodes have been collapsed/expanded
Replace null by undefined (to rebase)
Fix the item placement when copying.
If copying into a subtree or above the copied element, just insert the
element at the correct index.
If copying in the same tree and under the copied element, increase the
index by one to take account the presence of the orignal element
Ensure that a node has been given when generating dragInfo
Add new callback for entering/leaving copy mode.
Also apply the changement of the mode so it can two way binded

The status of copy mode is accessible via "$treeScope.copy"
Add a callbacks for lock/unlock of an axis (if key is used).
It receive the axis ("X" or "Y") as parameter.

Also apply the value so it can be two way binded.
The lock status is available via "$treeScope.lockX" or "$treeScope.lockY"
#160: Add the ability to multi-select elements.
Attention: drop the horizontal sorting capability. Will be re-added soon!

Simply configure the parameter "select" with the key of your choice and use
this key to toggle the multi-select behaviour.
Also add callbacks when entering/leaving the multi-select mode.
Also add callbacks when selecting/unselecting element. These callbacks
receives the selected/unselected node.
Also publish the status of the multi-select mode so that it can two way
binded. It can be accessed via "$treeScope.multiSelect".
When selecting a parent, it unselect all selected childs.
When selecting a node, if it parents is already selected, the node is
unselectable
It also smoothen up the position calculation (both horizontal and vertical)

It is possible to copy a multi-selection!

Available keys are: space, enter, tab, esc/escape, backspace/back, shift/
maj, ctrl/control, alt, left/leftArrow, up/upArrow, right/rightArrow, down/
downArrow, ins/insert, del/delete, home, end, pgUp/pageUp, pgDown/pageDown,
F1-F12, and any number/letter of the keyboard (except for the keypad
numbers).
Simply use the special key string or the letter/number (in any case) in the
"copy" parameter and you're good to go.
Customize the default style for a selected element
Add a small margin between elements and placeholders
Customize the cursor when in select mode
Customize the cursor when in copy mode
Fix indentation in the CSS file
Update the demo to use copy and multi-select mode
"CTRL" is the key for multi-selecting. "SHIFT" for copying.

Also change the style when in either of this two modes and when an element
is selected
Remove unused class
Rewrite parameters management.
Now every scope is isolated and every attribute is two way binded and
watched
Update demo to use and to demonstrate new parameters
Add an custom style to original element being copied.
Also customize the selected style
Remove a console log and fix code style
Fix position detection when going up.
Remove childs from the height calculation