@invariant supports use with no arguments

[mlhaufe]

Overview

Method decorators are registered but not active until this class decorator can associate them with the contractHandler and/or enforce their usage. This requirement simply allows an empty @invariant decorator to be used.

@invariant
class MyClass {
...
}

Tasks

• Implementation
• Implement overload so that parenthesis is not required
• Implement Unit Test
• Update README

[mlhaufe]

Subsumed by new architecture #171