Make sure that the cursor and text appear in the correct position on iOS 7

It appears that the default contentVerticalAlignment was "top" in iOS 6 and earlier,
and "centre" in iOS 7, so we just explicitly set it to "top" since that's what the layout
code in TITokenField expects it to be.
Merge pull request #9 from sadlerjw/fix-text-align-ios-7

Make sure that the cursor and text appear in the correct position on iOS 7