add warning message to email field on person edit page

#282 

This is how the field label looks like now:
![selection_004](https://cloud.githubusercontent.com/assets/2894750/4498624/f5428c6e-4a79-11e4-9853-a9f86157619f.png)

@georgebejan I was thinking we should make the I18n key for this something like `people.edit.email_hint` to be more consistent with some of the other help texts. Can you adjust the key name?