Simplify the serialization format

Simplifes the serialization format by getting not pickling the payload
separately.
Raise exception if no serializer found

Raise a SerializationError exception in cases where no adapter has been
registered for the object instance type to serialize.
Remove get_key() method from versioner and key_versioner
Adapt tests for key_versioner/versioner get_key() removal
Implement master-version feature in redis to handle version store unavailability
Adapt and improve tests a bit
Changelog
Merge pull request #6 from deuxpi/simplify-serialized-format

Simplify the serialization format
Style and opinion
Change exceptions naming convention (Failure -> Error)
Add unittest for Redis cache client
Merge branch 'master' of https://github.com/pior/pyramid_caching into pior-master
Merge branch 'pior-master'
merge
merge
Merge branch 'deuxpi-hit-miss-events'
remove umemcache in example
Fetch the metrics utility from thread-local data

If the cache events do not have access to the current request object,
get the current metrics utility directly from pyramid_metrics, which
would get it from a thread-local data structure.