Support Time for pointStart

At the moment it is not possible to set any point in time for pointStart. It always removes the time information. This PR would allow using a Time object.

Remarks:

• There was one test that uses an integer for pointStart (and a wrong value for pointInterval). Integers are not supported. I changed that test.
• Some tests on master did not work locally. It seems there is a problem with Webrat and RSpec

Failed examples:

rspec ./spec/lazy_high_charts_spec.rb:38 # HighChartsHelper layout_helper should return a div with an id of high_chart object
rspec ./spec/lazy_high_charts_spec.rb:42 # HighChartsHelper layout_helper should return a script
rspec ./spec/lazy_high_charts_spec.rb:49 # HighChartsHelper high_chart_globals should return a script
rspec ./spec/lazy_high_charts_spec.rb:62 # HighChartsHelper high_chart_graph ready function should be a javascript script
rspec ./spec/lazy_high_charts_spec.rb:67 # HighChartsHelper high_chart_graph ready function should assign to the onload event