Add flag to check whether an item in the Itemset is currently being dragged

Perturbing the nodes was being performed on all nodes regardless of whether they were set as fixed.

Added a "showLegend" option which can be used to toggle legend drawing
Added legend drawing to 11_tooltips and to playground examples
Added description of showLegend option to documentation

Signed-off-by: André Martins <aanm90@gmail.com>

It was broken by PR (#1729), which fixed issues #112 & #1730.
As far as I can tell that change shouldn't influence the DataAxis (vertical-axis), but only the horizontal axis.

 @yotamberk can you check if your examples still work as expected?

As the documentation on vis.js nodes can be styled with align: left|center

allOptions does not include this option and throw an error on option validation.

- Added interpolation option for interpolation of images, default true.

Internet Explorer does not support the ES6 Number.parseFloat (see http://caniuse.com/#feat=es6-number). Instead, substitute the regular parseFloat method.

- Added parentCentralization option for hierarchical layout.

instead of `'x'`, `'y'`, and `'z'`