Added Milestone Lines to Gantt Chart

hey bastian,

i added a parameter for milestones. i used your library for showing a quick roadmap of features, but what i needed was a marker for milestones over the year. So i modified the constructor to accept milestone dates and added a functionwhich draws a line on each milestone date.

My IDE also removed the whitspaces at the the end of a line which was actually just an accident, but i
think it is also useful because of saving bytes :-).

this is my first pull request so be gentle ;-).

Cheers,
Roland