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.

I'm not sure why do we use a ```String.Format``` here, isn't this whole thing equivalent to:

```cs
highlightingRule.Regex = new Regex(number + exponent);
```