Update the Terms of Service button color when creating a WordPress.com account

I am not totally sure this is the right way to attack this keeping in mind that the string needs to be translatable.
cc @astralbodies 
Fixes #1645

![ios simulator screen shot sep 19 2014 5 03 02 pm](https://cloud.githubusercontent.com/assets/363753/4343218/63afd702-4051-11e4-9b26-29cfb361f3f4.png)

Translations are tricky, like you said. I'm not sure we can trust that the `terms of service` should always be appended at the end of the string in every localization. In Japanese, for example, the verb would be last. 

An alternative approach might be to translate the complete string: `By creating an account you agree to the fascinating Terms of Service` and also translate `Terms of Service` separately. You could fine the range of the translated substring in the translated full string.  Then you could use `NSMutableAttributedString addAttribute:value:range:` to set the foreground color of the substring, and avoid having to fuss with concatenation.