Improvements in d3-gantt for high number of tasks

Addition of the following features in d3-gantt that are useful for high number of tasks :

 * allow of zooming on gantt chart
 * allow of tip on chart task
 * allow of click on chart task
 * parametrization of rectangle tasks rounding corners
 * display of horizontal lines for tasks possible
 * automatic computation of taskTypes
 * automatic resize of y axis font size

Done also a few clean up of code with indentation.

Added a sample with lots of tasks and all options activated.