Added custom timeline item labeling for easy modification

Each timeline item rectangle (or circle) is now affixed with a unique
id attribute (“timelineItem_”+customIdString) for easy manipulation
using JQuery. If no custom id is provided, the unique id will use the
data index, e.g.: timelineItem_0.
Added paragraph to describe it in the Readme.
Corrected two typos.
Merge branch 'master' of https://github.com/charlesfracchia/d3-timeline