Added EncryptedUSSocialSecurityNumberField to built-in encrypted fields.

Went ahead and merged pull request #26 into my fork.

Original Request:
I went ahead and added EncryptedUSSocialSecurityNumberField using the same technique as the EncryptedUSPhoneNumberField. Thought it might be convenient to have it built in, rather than force everyone who needs the functionality to roll it on their own each time.