Added support for PHP interface definitions to grammar
Added support for inheriting constants from parent class
Added support for referencing other class constants of the current class in the value of one

- eg. 'const A = self::B;'
Added basic support for interfaces to PHP interpreter

- Contains fatal error checking for attempting to define instance or static properties for interfaces
Raise correct fatal error when attempting to define instance or static method for interface with body
Added support for PHP class 'implements' to grammar
Added support for retrieving constants defined in interfaces implemented by a class