Added 'pattern' attribute to text field for iOS.

The keyboard shown for a text field can be affected by its pattern
attribute on iOS. See Apple's documentation for more details.

http://developer.apple.com/library/ios/#documentation/StringsTextFonts/Conceptual/TextAndWebiPhoneOS/KeyboardManagement/KeyboardManagement.html
Merge pull request #1946 from mutewinter/add_pattern_to_text_field

Added 'pattern' attribute to text field for iOS