Switch event creation method

Create events by selecting a region and pressing either enter or ctrl-n
Changed .background to .marker for timeline markers
Fixed bug that prevented from dragging backward to deselect

Place selection rectangle underneath background rectangles, hence the
low opacities
Commented out svg
Set duration of new event based on selection width
Prevented users from creating an event less than 30 minutes long
Placed events in proper row on creation
Merge pull request #50 from StanfordHCI/jf-taskRedesign

Switch event creation method