Adding writable as an option on property.

[fulrich]

By default property creates both an accessor and a reader for the property. Currently if you do not want a public accessor you have to do something similar to the following:

class TestClass
  property! :id
  protected :id=
end

This proposed change allows the ability to make a property read only:

class TestClass
  property! :id, read_only: true
end

When using the read_only flag as true an accessor will not be created for that property. If no read_only flag is provided the current default behaviour of creating an accessor will continue.

[t6d]

• Should we call the configuration option writable instead of read_only?
• Should we define a setter and make it protected rather than not defining it at all?

(I don't have strong opinions on either.)

[fulrich]

I liked the change to writable and have updated this PR to include it. I decided to keep the setters as not defined instead of protected simply because the naming doesn't convey this as strongly to me. writable states whether it is able to be written to or not. It doesn't convey that is can be written to but only by the class itself or inherited classes.