Added Milestone Lines to Gantt Chart

[rfiala]

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

[cycloon]

The commits are a little bit difficult to read because of your whitespace changes.

[arpitjacob]

couldn't figure out how to use the milestones could you update the sample code and help doc.

[n7st]

If you didn't figure it out yet here's an example, arpitnj:

$gantti = new Gantti($data, array(
    'title'      => 'Work List',
    'cellwidth'  => 25,
    'cellheight' => 35,
    'milestones' => array('2014-07-23', '2014-07-25')
));

Thanks for this extension!