Add forward slash to list of characters allowed in a style value

Hi - we are in the process of updating our app's version of NUI from 0.3.5 to the current 0.5.4. It appears that one of the changes between the versions removed the ability to have forward slashes (`/`) in image names within the stylesheet. 

Rather than storing our app's image assets in the root of the bundle, we store them in an "Assets" subdirectory. For example: `<AppBundle>/Assets/image.png`. 

So, with 0.5.4, styles like this were throwing an exception when parsing:

    Account_NavBar {
        background-image:    Assets/navbar_ios7.png;
    }

For my use-case, I'm able to add support for this by adding the forward slash to the list of valid `identifierCharacters` in the Identifier Token Recognizer. Are there any use-cases I'm not thinking about which might break something for others by adding this in? 

This seems loosely related to Issues #57 and #88. It doesn't resolve either of them, but this support would probably be a prerequisite for both of them.

Thanks!