Implemented support for date+time in TimeSlider

Rendering date+time if the range is less than a day but still spanning
over two dates. Add tests for the previous cases and clarified code to
make it easier to read.
Use strict comparison between dates
Fix timeslider mobile layout to fit date+time
Merge pull request #212 from CartoDB/211-fix-viz-torque-date-time

Implemented support for date+time in TimeSlider