FIX: Signature preparation niggles and bugs

[carlwilson]

• refactored ./fido/update_signatures.py to add clarity;
• generated FIDO sig file now validates against fido/config/fido-formats.xsd
  • fixed inconsistency between FIDO sig schema and 'fido/prepare.py''s use of 'apple_uid' and apple_uti;
• PRONOM HTTP methods in dedicated ./fido/pronom/http.py module (this needs to get more methods or merging with the SOAP module);
• added basic (hacky) catch for 404s when downloading test resources;
• safer use of empty list default for fido/prepare.py FormatInfo:__init__.

[carlwilson]

@sevein this is still a draft until I merge my previous PR but shows where I'm headed and the next set of changes.