Added support for declaring classes as abstract to prevent instantiation

git-svn-id: http://svn.code.sf.net/p/angelscript/code@1975 404ce1b2-830e-0410-a2e2-b09542c77caf
Improved error messages

git-svn-id: http://svn.code.sf.net/p/angelscript/code@1976 404ce1b2-830e-0410-a2e2-b09542c77caf