Don't require `is shared` for `def main`

[Chuck]

If a class has a parameterless initializer (whether explicit or implied) then don't require that the main be shared:

class Hello
    def main
        print 'Hello'

Generate an error if the class does not have a parameterless constructor.

If there is more than one main method in a program, that's an error due to ambiguity.

Also, provide a command line option to specify which method is the main method like C# does. In which case there is no ambiguity.