add textfield support

resolves #20 
resolves #24

Set alertViewStyle = SIAlertViewStylePlainTextInput to show a textfield in the alert

Retrieve the entered text in your callback with the inputText property

Needs some more work to be able to style the textfield, though UITextField seems to have no UIAppearance support. Maybe a solution would be to expose the textfield to make it configurable from outside?