AutocryptHeader.from_string
Description:
Creates a new AutocryptHeader object based on the value of an Autocrypt: header.
Note that this will not have an effective_date set, since the effective_date is derived from the Date: line in the 
      same block of e-mail headers, but cannot be extracted from the raw Autocrypt: header itself.
Parameters:
| string | 
           The raw string value of an Autocrypt header  | 
      
Returns:
| 
           a new AutocryptHeader object, or null on error.  |