feat(labels): Extract labels as a plugin

This implements #245 

http://rawgit.com/angular-gantt/angular-gantt/labels-plugin/demo/dist/index.html

Work in progress. ~~Rendering is crappy when labels plugin is not defined~~ OK now.

~~Maybe layout should be changed to be a table like (display: table, display: table-row, display: table cell) to layout the 4 main components : header, corner, side and body.~~ Was a bad idea :p

sortable plugin work as before (labels plugin raise directives `gantt-row-labels` and `gantt-row-label` events).