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.

Is this taken from the source code of ```HighlightingBrush``` class? I'm curious to know if there is a reference that this is patterned after, if so we may want to supply the link here? I ask that because I am not sure how to validate this implementation (apart from the fact that it works) Minor thing, though :)