Fix for highlighting of digits in code block

This fixes the issue shown below - any '.' before a digit (even if it occurs in a range expression) was considered as a decimal point of the number. 
![image](https://cloud.githubusercontent.com/assets/5710686/4330447/b910b966-3faf-11e4-8a2c-1c7b15e70fc7.png)

I have added a custom regex as per DesignScript ATG production rules for proper identification of digits in the syntax highlighter.

@Benglin please review. Thanks.

Taking back what I said about not using a converter (I was seeing that you instantiated a new instance of ```ColorConverter```, but I was wrong), this is actually very good! Thanks, I learnt something new :)