Provide retain annotation for properties

[jokade]

To simplify memory management, we should provide a @retain annotation for vars, similar to Objective-C:

import cocoa.foundation._

@ScalaObjC
class Foo(self: NSObject) {
  @retain
  public var date: NSDate = NSDate.alloc().init()
}

should be tranformed into

@ScalaObjC
class Foo(self: NSObject) {
  private var __v_date: NSDate = NSDate.alloc().init()

  def date: NSDate = __v_date
  def date_=(o: NSDate): Unit = {
    __v_date.release()
    __v_date = o.retain()
  }
}

[jokade]

Implemented by jokade/scalanative-obj-interop#9