property to enable/disable Done button

Hi Michael,

For my application I would like to use MWPhotoBrowser as the primary presentation UI. In other words, I need the photo browser to be the first screen that appears. The app will then proceed to display a collection of photos that were taken recently from Mars.

In my case, it is fine that the browser is parented by a NavigationController, but I never need to navigate "back" or click "Done" to dismiss the modal presentation.
To that end, I'm sending you this pull request to add a feature that I need. I added property to enable or disable the Done button when presented modally, enabled by default. When the property is set, the browser will skip the creation of the Done button.