Add support for CCLabel which uses System Fonts instead of Font XNB Content.
Add support to CCLabelTTF to use the Native font for labels if the font can not be found.
Only add SystemFontTest for Mac and iOS
Merge pull request #232 from mono/sysfontlabel

Add support for CCLabel which uses System Fonts instead of Font XNB Content
bug fixing to remove legacy code that checks the index against the count of layers which breaks the use of the tag as the index reference.