Add optional getter and setter support for classes

If the transform option `es5` is set to `true`, the class transform will now use `Object.defineProperty(...)` to create getters and setters. Line numbers are preserved.

Example input:
```
class Foo {
  static get answer {
      return 42;
  }

  get title() {
     return this._title;
  }

  set title(value) {
    this._title = value;
  }
```

Example output:

```
function Foo(){"use strict";}
  Object.defineProperty(Foo,"answer",{enumerable:true,configurable:true,get:function() {"use strict";
    return 42;
  }});

  Object.defineProperty(Foo.prototype,"title",{enumerable:true,configurable:true,get:function() {"use strict";
    return this.$Foo_title;
  }});

  var ____FooDescr=Object.getOwnPropertyDescriptor(Foo.prototype,"title");Object.defineProperty(Foo.prototype,"title",{enumerable:true,configurable:true,get:____FooDescr.get,set:function(value) {"use strict";
    this.$Foo_title = value;
  }});
```
Oh wow... I thought it would overwrite the whole definition, i.e. would set either the getter or setter (depending on what's omitted) back to `undefined`. But it actually preserves the existing getter or setter. This will simplify things a lot!
I will make the changes.