When bool value is converted to string, string should be lowercase

This pull request is to fix defect [MAGN-3420 No consistent notation of true / false](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3420#). 

The problem is because to ``ToString()`` in .Net which converts ``true`` to ``"True"`` and ``false`` to ``"False"``. There are three cases that we may display string value:
  1. Watch window
  2. Preview window
  3. Default value in library view

So the fix is to change the string to lowercase if the type is boolean. 

![untitled](https://cloud.githubusercontent.com/assets/2600379/4058538/475f34bc-2dd7-11e4-8034-d073e4e33989.png)

@Benglin , please take a look. :-)