- add binding ZID/sipURI in cache to prevent evil friend attack
- expose read/write cache API for external use(LIME)
- add callback to give information to user(cache mismatch only for now)
- stick to RFC 4.5.2 for exported key generation
  (but use the ZRTP Client ID to ensure backward compatibility)
- direct access to exported key computation(not anymore related to cache)
- add cache related tests

Note: no cache migration function provided

- use libxml2 to manage cache
- add set/unset previously verified sas flag management interface
- clean debug code

+ remove peerSSRC from context as it was useless
+ get correct test patterns for parser/builder