adds option to only tick at block start

[jmbeach]

Adds an option to solve the feature request in issue #102.

Use the new option atBlockStart: true as part of the tickFormat method.

Ex:

d3.timeline().tickFormat({ atBlockStart: true })

Also adds hideOverlapTicks options (which only works with atBlockStart).

hideOverlapTicks is an amount of ticks to not allow two timestamps to be drawn next to eachother and prevents two timestamps from overlapping.

Ex 1: atBlockStart: true:

Ex 2: atBlockStart: true, hideOperlapTicks 2700000 (45 min)