Add support for abstract classes and methods

[MichaelHochriegl]

It would be nice to be able to make abstract classes and methods. Currently only virtual is supported.

My Fluent-API would look like this:

var builder = CodeBuilder.Create("AwesomeApp")
                .AddClass("SampleClass")
                .MakeAbstractClass()
                .AddMethod("MyAbstractMethod")
                .MakeAbstract()
                .Class;

These should produce an output like:

namespace AwesomeApp
{
    abstract partial class SampleClass
    {
        abstract void MyAbstractMethod()
        {
        }
    }
}

I've got a pull request ready for this, but am of course open for changes to this. My approach follows the virtual style to be consistent.