iOS: Initial implementation for ComboBox.  Still needs work on iPad (e.g. to use popover)
iOS: Adjust edges of non-scrollable views to show entire content of view
iOS: Adjust size of spinner based on available size for control
- Added Dialog.ShowModal() and Dialog.ShowModalAsync()
- Removed ModalWindow in favour of using Dialog.ShowModalAsync() to make for better. Fixes #197
- Obsoleted Dialog.DialogResult, and Dialog.ShowDialog()
- Updated Eto.Test projects to .net 4.5 to use async keyword