Grouping tasks phase 1

Grouping #59 . The tasks can belond to grouping tasks, which have
attribute 'group' set to true. Then, each subtask has to be included in
values array. Grouping task length is calculated basing on subtasks
dates. Task groups can be collapsed (static setting). Dependencies are
temporarily down.
TODO:  dynamic groups collapsing/expanding
dependencies working with groups
Grouping tasks phase 2: collapsing

Grouping #59. Now collapsing/extending tasks is ready for testing!
dependencies working with grouping tasks

Dependencies working again!
Hidden tasks have their parent task id stored in global array
(hiddenIds) - when we draw dependencies and group is collapsed, lines
are drawn between grouping tasks. Another contribute to #59.
Markup corrections (operators)

'==' to '===' and '!=' to '!=='
Styling, markup and minify

#59: styling for groups of task. Some minor markup corrections and
minify js file (with yui).
Merge pull request #3 from alekwisnia/groups

Groups