Dec num

For https://trello.com/c/zlnGJoCJ/134-decimals-on-the-number-line

Rename decimals_on_the_number_line_2.html (negative numbers), exercises/decimals_on_the_number_line_3.html.
Create new decimals_on_the_number_line_2.html - move the point to a decimal with a hundredth place.
Also the number line doesn't necessarily start at 0.

Tweak all the exercises to improve the hints and use standard colours.