在使用UITextField的时候，我们经常会有需求来控制打开的键盘的类型，虽然文档里都有相应类型的说明，但都不够直观。现在，把所有的UIKeyboardType来以图片展示下。可以直观的来选择所需的键盘种类。
UIKeyboardTypeAlphabet
UIKeyboardTypeASCIICapable
UIKeyboardTypeDecimalPad
UIKeyboardTypeDefault
UIKeyboardTypeEmailAddress
UIKeyboardTypeNamePhonePad
UIKeyboardTypeNumberPad
UIKeyboardTypeNumbersAndPunctuation
UIKeyboardTypePhonePad
UIKeyboardTypeTwitter
UIKeyboardTypeURL